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Fuel Efficiency and Slow Driving?

timothy posted about 6 years ago | from the practical-interest-here dept.

The Almighty Buck 1114

vile8 writes "With the high gas prices and ongoing gas gouging in my hometown many people are trying to find a reasonable way to save gas. One of the things I've noticed is people driving exceptionally slow, 30mph in 45mph zones, etc. So I had to take a quick look and find out if driving slow is helpful in getting better mileage. I know horsepower increases substantially with wind resistance, but with charts like this one from it appears mileage is actually about the same between 27mph and 58mph or so. So I'm curious what all the drivers out there with the cool efficiency computers are getting ... of specific interest would be the hemis with MDS; how do those do with the cylinder shutoff mode at different speeds?" Related: are there any practical hypermiling techniques that you've found for people not ready to purchase a new car, nor give up driving generally?

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Fuel economy (5, Informative)

JWSmythe (446288) | about 6 years ago | (#25348607)

I spent some time researching this matter after a discussion at work started about it.

Something that I had observed in my car was that my fuel economy increased as my speed increased.

At a cruising speed of 85mph, I get 26mpg. at 80mph, I got 24mpg. And at 65, i got about 20mpg. This testing was done along I-10 between Jacksonville and Los Angeles. There's lots of room to set the cruise control. A test usually consisted of fueling up, then a hard acceleration to the testing speed and setting the cruise control to handle maintaining the speed for the next 300 to 350 miles. Individual tests were spot checked (repeated somewhere else on the drive).

In researching this, it wasn't a matter that my car is "faster", stronger, or just plain cooler. It's a function of the drag of the vehicle and the RPMs the engine is turning.

Most cars make their best fuel economy somewhere between 1800 to 2200 rpm. Ah ha! My car has a 6 speed stick. If I'm in 6th gear it's turning about 2000rpm at 85mph.

I then compared ground speed to engine speed ratios of other cars, partly selected because they were owned by people in the discussion, or because they were fairly common cars. Depending on the vehicle, it's best cruise speed could be anywhere between 45mph to 90mph.

Re:Fuel economy (1, Interesting)

Ethanol-fueled (1125189) | about 6 years ago | (#25348667)

The key is to drive a manual transmission and to hold in the clutch whenever you can(especially downhill) so that the car coasts(runs at idle) as much as possible.

And don't engine brake because that is poor form and is retarded. Use your damn brakes.

Re:Fuel economy (5, Insightful)

JWSmythe (446288) | about 6 years ago | (#25348695)

    I've gotten lazy with this. When I know a stop is coming up, I put the car in neutral and let it coast. After a while, my clutch leg starts to hurt, so it's easier this way. People may think I'm weird, but I do it up to a mile away. The car coasts really well, so I'm usually not going any slower than I should be anyways. :)

Re:Fuel economy (5, Interesting)

Goldberg's Pants (139800) | about 6 years ago | (#25348727)

I would always just leave the clutch in rather than put it in neutral in case you have to get out of trouble.

I remember once, on the M27 in England, a friend and I were driving to Portsmouth. We'd be put just over a pounds worth of petrol in. Gauge didn't even register empty. It was below that. There's a long downhill bit heading down into the city. He put the clutch in and we coasted several miles at least. (This was in a really old Saab.) Little disconcerting, especially when venturing onto a Motorway in the knowledge that your car may break down is an offense.

Re:Fuel economy (0, Flamebait)

techno-vampire (666512) | about 6 years ago | (#25348861)

Little disconcerting, especially when venturing onto a Motorway in the knowledge that your car may break down is an offense.

Well, isn't that convenient for the bobbies? Especially when you consider that any car can break down at any time and every driver should be aware of that.

Re:Fuel economy (5, Informative)

cailith1970 (1325195) | about 6 years ago | (#25349023)

As an ex-mechanic, I wouldn't recommend coasting all the time with your clutch in, you're not doing it any favours. Stick the thing in neutral, it's far better for the longevity of your clutch, not to mention your spigot bearing.

Re:Fuel economy (5, Funny)

mustafap (452510) | about 6 years ago | (#25349093)

>not to mention your spigot bearing.

Is that something a doctor would sort out, or a mechanic?

Re:Fuel economy (5, Informative)

JohnnyBGod (1088549) | about 6 years ago | (#25348739)

Engine braking is a fine way to brake, nowadays. Modern engines don't burn any fuel while engine braking, and the braking is usually slow enough for the unlit brake lights not to be a problem.

Plus, it's essential while going downhill, otherwise your brakes will be useless. Unless what you want to accelerate, of course. Then, by all means, let gravity help you.

Re:Fuel economy (4, Informative)

rwade (131726) | about 6 years ago | (#25348779)

He's not referring to engine breaking down a hill or simply allowing the engine to slow down the car in whatever gear the car is in. He's referring to those that will downshift, engage, downshift, engage all the way to a light.

Re:Fuel economy (-1, Flamebait)

Creepy Crawler (680178) | about 6 years ago | (#25348801)

Engine breaking on vehicles that are not intended for it (most passenger vehicles) cause ecessive wear and tear on the engine.

Using brakes wears out the brakes. Brake pads are 20-30$.
Using engine braking wears out the engine. Engine overhaul is multiple thousands $.

Guess what I'd rather repair.

Re:Fuel economy (4, Insightful)

base2_celtic (56328) | about 6 years ago | (#25348851)

Foolish. All engines are designed to brake.

Brakes are a safety device; you need them for keeping the car stationary, bringing it to a stop from low speeds, to prevent emergency situations from occurring, and for use in an emergency situation itself.

You save them; you don't use them. If you use your brakes as a matter of course, to control your speed, then you won't have them when you need them, due to heat build-up. No, disc brakes won't save you - they're better than drum, but they still heat up and loose effectiveness. No, ABS won't save you, either. It still has nothing to play with if your brakes have heated up.

Which would you rather: a bit of fuel economy (dubious anyway), or a large funeral bill?

Re:Fuel economy (-1, Redundant)

aliquis (678370) | about 6 years ago | (#25348917)

Yeah, if it was bad to engine brake why wouldn't the educations to get a driving license inform on that and why would they want you to do it as much as possible?

Anyway, funny to see what is considered low gasoline consumption and environmental friendly over seas :D.

1.67 $ / litre over here, or 6.33 / gallon.

And kind of all "normal" new cars do around 0.7 or maybe 0.8 don't they? Except Volvos and Saabs (I'm swede ..)

So i guess getting something japanese and more gasoline efficient would be a first step.

But in the end electric wins so hard.

Re:Fuel economy (4, Funny)

aliquis (678370) | about 6 years ago | (#25348923)

Btw it wouldn't be my funeral bill would it!?!

Re:Fuel economy (1)

PachmanP (881352) | about 6 years ago | (#25348925)

My brakes are carbon ceramic, you insensitive clod! If I don't brake hard and often they get cold and don't do anything to stop me!

Re:Fuel economy (5, Informative)

Sique (173459) | about 6 years ago | (#25349033)

You obviously don't live in the mountains. Here not using the engine for braking is a sure recipe to roast your brakes and have a pedal response like pudding after 10 mls, and yes, we here have slopes of 20 or 30 mls. Coming down from Timmelsjoch to Haiming we start out at ~7500 ft and after nearly 6000 ft we reach Haiming at an altitude of 1800 ft. If you ever plan to do that with only the brakes please send me a message upfront, so I can avoid driving there that day.

I am using my engine for braking all the time, it has now 80,000 mls, no sign of wear and tear, and I have replaced the brake discs once.

Re:Fuel economy (5, Informative)

MentlFlos (7345) | about 6 years ago | (#25348741)

The key is to drive a manual transmission and to hold in the clutch whenever you can(especially downhill) so that the car coasts(runs at idle) as much as possible.


If you are at 0% throttle and over a certain RPM most ECUs will turn the injectors off (0 fuel usage). I am sure you will agree that (fuel used to idle the the motor) > (no fuel used).

Should I even bite on your username or just let that one slide?

Re:Fuel economy (0, Troll)

aliquis (678370) | about 6 years ago | (#25348943)

Uhm, wouldn't it "idle" in both cases? Or why would it required more if you pressed in the clutch? Because the software "believes" it have to be ready?

Re:Fuel economy (1)

JWSmythe (446288) | about 6 years ago | (#25348995)

    You must really be smoking crack.

    If the injectors turn off, the engine doesn't continue to run. It's kinda dependent on getting fuel,duh. Now, an idle motor can run for an awful long time on a lot less fuel. We ran one car at idle with the A/C on (special purpose, don't complain), and what was a 2500 mile drive and 8 tanks of fuel in the vehicl doing the towing was about 1/8 of a tank in the one idle on the trailer. That was about 2 gallons for 2500 miles, or 1250mpg. :)

    Engine breaking does bring the RPMs up, but the throttle is usually closed and the injectors are just spraying enough to keep it going at the given RPM. Being that you had to use engine braking means that you consumed more fuel than you needed to prior to braking. Your using some method to bleed off speed. If you shifted to neutral sooner, and made a comfortable slow deceleration, you would save fuel.


Re:Fuel economy (1)

caramelcarrot (778148) | about 6 years ago | (#25348743)

This is a dumb and counterproductive suggestion. Keeping your gearbox engaged as you run down a hill will keep your engine turning, meaning your car won't have to use fuel to keep it above stalling speed. Sure, go to a higher gear, but don't disengage entirely. Remember that breaking is lost energy.

Re:Fuel economy (1)

rwade (131726) | about 6 years ago | (#25348785)

Additionally, this is a good time to turn the A/C on, since the power it draws will take some of the car's speed away and basically for free if the hill is steep enough and long enough.

Re:Fuel economy (2, Informative)

Gordonjcp (186804) | about 6 years ago | (#25348749)

The key is to drive a manual transmission and to hold in the clutch whenever you can(especially downhill) so that the car coasts(runs at idle) as much as possible.
Doesn't help with fuel economy, and will very *very* quickly destroy your clutch release bearing. If you *must* do this, put it in neutral. The 5p worth of petrol you save probably won't offset the cost of pulling out the engine and gearbox to replace the clutch...

And don't engine brake because that is poor form and is retarded. Use your damn brakes.

Makes no difference to fuel economy. I suspect that using the footbrake rather than engine braking will actually make you use *more* fuel. On damn near any European car built in the last 25 years, the engine will use no fuel at all when you're engine braking.

Re:Fuel economy (1)

gnupun (752725) | about 6 years ago | (#25348807)

The key is to drive a manual transmission and to hold in the clutch whenever you can

You can do the same with an automatic by switching from "D" to "N" (neutral). But I've heard that's not good for the transmission. You also lose engine-braking, as a safety issue.

Re:Fuel economy (1)

aztracker1 (702135) | about 6 years ago | (#25349047)

I like that my car is an automatic, but also has a manual shift mode.. :D best of both worlds... I live in phx during the week for work, but my family & home are about 3/4 of a mile higher in elevation... going downhill and being able to select the gear is better than letting it coast full out in neutral.

Re:Fuel economy (1, Interesting)

aliquis (678370) | about 6 years ago | (#25349061)

May someone tell me if you are supposed to switch over to N on an automatic if you stand still at say red lights or such for a short while or should you just release the gas and break?

Re:Fuel economy (4, Insightful)

base2_celtic (56328) | about 6 years ago | (#25348817)

Holding in your clutch a lot will stuff your throw-out race. That's really bad, because that'll lead to a clutch that you can't engage or disengage. Not having a clutch when you need it is really really bad.

Engine braking is good practice. Putting aside the cleverness of modern ECUs, most footbrakes fade with use as they heat up. Yes, even disc brakes; they're a heap better than drum, but they can still overheat very rapidly.

If you're coming down a very long, steep hill and you're not engine braking, your brakes will be much less effective by the time you reach the bottom.

Brakes are a safety device, not a speed control tool. You brake only when required. Your accelerator pedal and engine braking is what you use to control your vehicle's speed. If you need to use your brakes other than to come to a stop, you're pushing your car too damned hard, or you're too damned close to the guy in front of you. Back off.

Re:Fuel economy (1)

Fluffeh (1273756) | about 6 years ago | (#25348831)

Not to be rude, but informative != wrong. (Maybe wrongly informative?)

If you value your clutch, do NOT sit with your foot on the clutch while rolling down a hill. Quickest way short of riding it to need a replacement.

Also there is utterly NOTHING wrong with using your engine to brake with - especially over long distances/downhill sections. Firstly is stops your brake pads getting overheated and burning out, secondly it doesn't use more fuel at all it is in fact the lack of fuel available that causes the vehicle to slow down.

Man, I don't consider myself an expert on cars, but I would dearly love to be your mechanic.

Re:Fuel economy (2, Informative)

rwa2 (4391) | about 6 years ago | (#25348833)

Driving in neutral or with the clutch engaged is also illegal in many states.

That said, I do do it sometimes when the coast is clear and it looks safe. It's a bit harder now that I usually drive an automatic... have to get the rpms at the right place before putting it back in D ... never got my wife to learn to drive standard).

Also note that engine braking is absolutely necessary in mountainous / hilly terrain. When your brakes overheat from riding them down a long hill, they kinda stop working. Then your wheels catch fire.

But when it's safe to do so, certainly use your brakes rather than your engine. Engine braking doesn't really use fuel, but it's better to wear down your brake pads as opposed to wearing down your engine.

Ideally, you want to try to drive such that you don't use your brakes at all. After all, braking means you're converting your kinetic energy into heat (unless you're lucky enough to drive a hybrid with good regenerative braking). If you never brake, then that means you're never hitting the gas more than you need to, so you're not wasting.

Anyway, if you also make sure your tires are inflated properly, and make sure you always drive in the highest gear you can as much as possible, you should already be in pretty good shape.

Re:Fuel economy (1)

Dare nMc (468959) | about 6 years ago | (#25348837)

I am curious what you drive? I drive a diesel pickup on a 20 mile trip to work with a large hill in the middle. My overall mileage using neutral down hill went from 19MPG to 23MPG (actual overall, the MPG gauge went from 20 to 26 overall.) Doing the same in a 4 cylinder 60hp car made little or no difference to it's MPG. Then again my truck idles 700rpm, 6th gear 55 mph is 2100 rpm, where the turbo starts kicking in. her car goes from 1200rpm in 5th to 900 rpm idle (maybe I need to kill the AC as well.)
I noticed some newer (pickup) automatics shifting out of lockup while costing at speed, so again neutral at stop lights still helps, not sure neutral while moving is needed for many vehicles.

Re:Fuel economy (0)

Anonymous Coward | about 6 years ago | (#25348869)

If you must coast in neutral, take the car out of gear and release the clutch. Holding the clutch down will wear out the clutch throw out (thrust) bearings. These bearings are designed to take the momentary load of a clutch depression, not long term load. Coasting in neutral is a dangerous driving practice, as, the car is effectively out of control.

Re:Fuel economy (1)

TallGuyRacer (920071) | about 6 years ago | (#25348889)

Wrong. Most modern cars will shut the injectors off completely when you take your foot off the accelerator and the engine is operating above idle RPM. When the engine is idling with the clutch in, it has to be supplied with a small amount of gas to keep it turning over and prevent it stalling.

Re:Fuel economy (1)

ptbarnett (159784) | about 6 years ago | (#25348787)

Something that I had observed in my car was that my fuel economy increased as my speed increased.

I've experienced the same thing, although my sample size is still too small to be conclusive.

If I can sustain 75 mph (which is possible in parts of my state), I seem to get about 29-30 mpg. If speed limits require me to drive slower (60-65), I get 26-27 mpg.

Re:Fuel economy (2, Interesting)

// (81289) | about 6 years ago | (#25348911)

You're proud of 30mpg?

I can commute about 40 miles to work in the rush hour traffic and get over 50mpg without even trying.

Outside of rush hour, a leisurely cruise back along the same route easily gets 60mpg. Best I've ever achieved was 77mpg, but that was actually quite a lot of work. It's not constant speed that you want for maximum mpg, and although using a cruise control gives a reasonable zeroth order approximation, you can do better. Of course, if the road is 200 miles long, with no variation in incline or corners, and no traffic causing you to slow down unnecessarily, then cruise control will equate to the best mpg you can get :-)

If I drive like a nutter, I expect to average around the 26mpg.

Maybe you guys need some European cars? You'd certainly not be satisfied with 30mpg at our current fuel prices ($2/litre)....

Re:Fuel economy (1)

aztracker1 (702135) | about 6 years ago | (#25348985)

I just bought a new 2009 Hyundai Sonata, and seem to get my best milage between 60-80mph. It's actually better at 80, than it is at 45. If I drive like an 80yo man wrt acceleration, etc around 65mph, I get around 34.5 mpg on the city highways in the Phoenix area.

Re:Fuel economy (1)

reeeh2000 (1328037) | about 6 years ago | (#25349017)

Simply not peeling out of traffic light has saved me a considerable amount of gas. I've got a 97 Dodge Dakota 3.9L V6. I find that by trying to maintain under 2000 rpm while driving and 2500 while accelerating has a huge affect. My advice, watch your tachometer first. Figure out where it sits at different speeds and adjust your driving accordingly. My other tips include basics such as frequent oil changes and making sure that your tires have the right pressure. Don't fall for any of the gimmicks like fuel4gas. If you want to make an improvement that you can actually afford, for about the price of a tank of gas you can drop in a K/N air filter. I'v got one and can tell the difference when I accelerate. Finally, never buy FRAM.

I don't own a car. (3, Funny)

Anonymous Coward | about 6 years ago | (#25348609)

But I've noticed I get less tired if I walk rather than run :-)

Sammy Hagar Says: (1)

SpeZek (970136) | about 6 years ago | (#25348619)

Tag for this one: icantdrive55

Re:Sammy Hagar Says: (1)

spazdor (902907) | about 6 years ago | (#25348945)

Schfifty five?

Its not fuel economy, its self righteousness (0)

Anonymous Coward | about 6 years ago | (#25348637)

The people I encounter who are going noticably slow in areas are using MPG as an excuse to be self righteous, and block lanes of traffic. Its just another ploy for some to see how much they can get away with annoying people.

Re:Its not fuel economy, its self righteousness (4, Insightful)

$RANDOMLUSER (804576) | about 6 years ago | (#25348763)

Yeah. There was an article in Readers Digest a few months ago about "hypermiling" or whatever, and it was a case study in "I'm the only person on the road". Incredibly arrogant and self-centered.

Re:Its not fuel economy, its self righteousness (0)

Anonymous Coward | about 6 years ago | (#25348875)

Right. For some people time IS money. Slowing them down by driving too slow will cut into their paycheck and really piss them off. If they happen to be driving a huge truck and you're in a tiny car, you could be endangering your life. Personally, I'm in favor of shooting ultra-slow drivers. :)

Re:Its not fuel economy, its self righteousness (1)

Daniel832US (530981) | about 6 years ago | (#25349099)

I'll admit that I'm slowing down to save gas, but I always do it in the outside lane. I've got a 2008 Scion Xd that's rated for 27/33 (automatic). By (a) starting off slower and (b) keeping it between 55-60 on the interstate, I can average 42-43 MPG for the tank (it resets itself every time you fill up). When starting off, I remember something I read about pretending your foot is on an egg on top of the gas/break petal. Don't push down too hard too fast and if you see a red light ahead, take your foot off the gas.
I'll go 55 on the interstate and have everyone speeding by, but I don't mind. Most of the stretch I'm on is rated 65/55 so the trucks are supposed to be going 55 anyway (which they don't). By the time it gets to the 70 MPH zone, it's starting up a big steep hill so the trucks are slowing down to 45 by the time they reach the top anyway due to gravity. Down the other side and I'm ready to exit.
I'm also lucky... the town I'm in had a new gas station come in and they're having price wars. It's down to 2.58 already for regular :)

I've never looked into this but... (0, Offtopic)

thetoadwarrior (1268702) | about 6 years ago | (#25348643)

I wish people would use logic when they over take people. There is absolutely no reason to over take under dangerous conditions and, unless you're on a motorway it won't actually help you.

Thrashing your car and risking everyone's lives for a few seconds will not make up for driving slowly for a few minutes. It's not hard to prove either as anyone with a GPS tracker can see how little over taking does for your over all time but the damage it can do to your engine (and maybe your life) are greater.

As far as fuel efficiency, I think maintaining a constant speed, whether it be 30 mph or 50 mph is more efficient than going up and down. Since most people usually vary their speed for things like over taking and beating red lights and they may normally drive slower than I suppose the fact their trying to be consistent, even if it's at an annoyingly slower speed, are benefiting.

My best mileage is around 55 mph (1)

NeverVotedBush (1041088) | about 6 years ago | (#25348661)

I've got a Honda Civic Hybrid. And have I ever gotten to love the mileage!

At 75, the mileage drops down to ~45 mpg. I haven't tried extended driving at lower speeds - which you really need to do to get a good handle on mileage so that you average out grade and wind factors.

But the biggest factor affecting my fuel economy was being able to look at the world situation, understand the concept of peak oil, and understand the effect that a developing world would have on existing fuel supplies - which led me to buy high mileage automobiles.

That is why I routinely get mileage in the high 40's to very low 50's.

Re:My best mileage is around 55 mph (1)

iminplaya (723125) | about 6 years ago | (#25349083)

I can't drive 55 []

Missing? (1)

Azarael (896715) | about 6 years ago | (#25348665)

The obvious answer that everyone seems to be ignoring is that you get the best fuel efficiency by not going anywhere at all. Thus we should all become hermits and all turn our cars into permanent drive-in theaters in the back yard. The stat I've heard before on fuel efficiency is 90km/h or ~50mph, but JWSmythe's comment makes sense also.

My plan for improving the world's fuel consumption (5, Insightful)

Joce640k (829181) | about 6 years ago | (#25348669)

Make those fuel consumption displays mandatory.

Most cars these days know their consumption - it's one of the first things they look at when they connect the laptop to the engine when you go for a service.

Make the display mandatory, make it large, and put it in a prominent place. It'll do wonders for everybody's fuel consumption.

Re:My plan for improving the world's fuel consumpt (5, Insightful)

Rogerborg (306625) | about 6 years ago | (#25348765)

Oh, for mod points. Most people (well, most men anyway) are competitive, and we like to beat our "high scores". Tachometers show us speed, clocks show us time, but neither of those contributes to efficiency. Adding a fuel economy display gives a better goal to beat.

BMW on fuel efficient driving (5, Informative)

base2_celtic (56328) | about 6 years ago | (#25348679)

An American Road & Track issue from many years ago (and I'm damned if I can recall which one) had a long article on the results of some fuel economy studies conducted by BMW.

The findings seemed to show that driving style was more important than overall speed.

The tips, in general, were:

- Keep your speed constant; fluctuations up and down are bad.
- Accelerate to your target speed quickly. Spending time slowly accelerating up to it wastes fuel.
- Be in the highest gear feasible for your engine type and road speed.
- 75% throttle for acceleration, conditions permitting.
- Keep your revs low, and change gears often to keep them low. That said, know your torque curve, and use it; if you have a small 4 cylinder, trying to accelerate at 1000 revs is futile.

Re:BMW on fuel efficient driving (4, Informative)

Lumpy (12016) | about 6 years ago | (#25348775)

Myth #1

Accelerate to your speed quickly. This actually wastes gas. It's usually touted by people that really dont know how cars work.

accelerate in your engine's economy band. this can easily be found by watching your MPG gauge or using a $12.95 Vaccuum gauge attached to your car's vac system.

Flooring it to your speed wastes gas, you are running rich the entire time putting fuel out your tailpipe. Going to slow wastes fuel as well, accelerate as to what your car's max economy is for that driving situation. problem is most cars are not equipped with the gauges needed to do this. American cars are designed for really stupid drivers, so they remove most of the gauges. too many gauges confuse american drivers.

75% is inaccurate for most cars. If I was driving a high performance car, 75% throttle is burning tires. In a smart car it's too little as it's power band from a stop is a gradual increase from 45% to 80% as your speed increases so you can keep the engine in it's power band for max economy.

Basically you have to learn your car. It takes time and efffort to maximize fuel economy. as well as getting rid of retarded driving habits like the morning dragracing from light to light. Accelerate slowly and time the lights to you never have to stop saves more gas than anything else.

Oh, Car and Driver reviewed those BMW tips, they found that they contradict each other.

Re:BMW on fuel efficient driving (1)

rwade (131726) | about 6 years ago | (#25348863)

Oh, Car and Driver reviewed those BMW tips, they found that they contradict each other.

Imagine that; a competitor to Road and Track questioning the authority of it's suggestions...

I'm inclined to agree that you're correct, but can you provide a better source?

Re:BMW on fuel efficient driving (1)

Smoke2Joints (915787) | about 6 years ago | (#25348913)

so basicly you mean... to drive fuel-efficiently, you need to drive fuel-efficiently. yea, that makes sense.

Take advantage of aerodynamics (5, Funny)

kbrasee (1379057) | about 6 years ago | (#25348685)

Do like everyone else does, drive about 6 inches behind me at 65 mph.

Thanks! (4, Funny)

ciaohound (118419) | about 6 years ago | (#25348933)

I always thought those people were assholes, and I'd fly into a rant about how dangerous and reckless that behavior was. But they're just trying to save money. People really are basically good after all!

Re:Take advantage of aerodynamics (1)

lysergic.acid (845423) | about 6 years ago | (#25348953)

hey, i'm not tailgating, i'm drafting!

Lower speed = Better MPG for me (4, Insightful)

glitch23 (557124) | about 6 years ago | (#25348689)

There are sweet spots for driving which is usually specific to the type of vehicle, the gearing, etc. so, to an extent, I'm sure the faster you go the better MPG you will see. But for my car, Mitsubishi Spyder, they recommend shifting into 6th at about 50mph. So basically my interstate driving is all in the top gear by far. At 70-75mph driving on WV interstate highways I get about 20-21 MPG. If I just drop my speed to 65mph everywhere I go during a tank of gas I can reach 24 MPG. I've consistently seen those results out of at least the last 3 or 4 tanks of gas over the last couple months. If I take a US Route (speed limit 55) for 90 minutes to visit my parents my MPG goes up even more for that period of time because I'm going even slower than my usual 65-75 mph. I don't drive too much slower than the posted speed limit (5mph as I state above) because I don't want to feel like I'm crawling but just dropping 5 mph makes a noticeable difference in the range I can achieve with my tank (17.7 gallons). YMMV.

It's about acceleration (5, Interesting)

Anonymous Coward | about 6 years ago | (#25348709)

I have a realtime mileage display and variable cylinder technology in my car, and what I have noticed is that I can easily cruise at 75mph on 3 cylinders and get tremendous mileage in the process. However, when I hit an uphill grade, if I try to maintain 75 the other cylinders kick in and my mileage drops dramatically (to roughly 2/3). But, I have noticed that if I gradually back off on the accelerator while climbing the grade, bleeding down my speed to keep those other 3 cylinders from turning on, I can climb the hill while maintaining my high mileage. I've learned also to accelerate slowly on level and mild up grades (like near the top of the grade) without the other cylinders engaging. Obviously when going downhill I take full advantage and build my speed back up while still getting great mileage. Perhaps something like this is what you are observing? BTW, I don't play these games in heavy or rush hour traffic; I only use these techniques when traffic is light.

Other helpful practices: smart braking (5, Informative)

Cordath (581672) | about 6 years ago | (#25348713)

There are lots of little things you can do to save on gas. Many center around efficient stopping.

For example, if I see a red light coming up, I'll often ease off the gas and coast in rather than maintaining speed and then braking near the light like most people do. In addition to saving gas on the way to the light, if the light turns green before you stop then you've also saved the gas it would have taken to accelerate back up to speed.

This tactic can be quite entertaining if, for example, an impatient bozo in a SUV comes up behind you while you're coasting, honks, pulls around you and speeds ahead only to stop at the light, and then you smoke him as you coast through the light just as it turns green.

Mod parent up. (3, Insightful)

seeker_1us (1203072) | about 6 years ago | (#25348891)

While wind resistance scales with speed squared, the simple fact is that most of the energy wasted in a car is in stopping, not wind resistance. Normal driving around the city I can get 19-22 MPG, and I use smart braking like the parent discusses. Driving 65-75 MPH across states (where I am just GOING), I can get almost 35.

Re:Mod parent up. (1)

Falstius (963333) | about 6 years ago | (#25349073)

Not to nitpick, but air resistance goes with velocity cubed, not squared. Coasting to stop lights is one of the best and easiest ways to save gas (along with not driving overly fast on the highway and moderate, smooth acceleration).

A comparison I can think of. (1)

smchris (464899) | about 6 years ago | (#25348719)

First 100 miles after filling the Prius and mostly interstate going 70-75 mph with moderate lane-changing in moderate traffic got 50.2. Did a back-highway trip this spring of 300+ miles mostly holding to the local 55-ish while driving "Priusy" and the mileage for that tank was showing 52.1 when we got home. So I'd say not a huge difference based on various top legal speeds.

Re:A comparison I can think of. (0)

Anonymous Coward | about 6 years ago | (#25349079)

As I'm sure you know, the Prius has regenerative braking which recaptures the energy lost while braking in city traffic. So the Prius tends to get better city mileage than most other cars. In fact I've even heard of some Priuses that get better city mileage than highway mileage.

Most typical cars don't have regenerative braking, and thus suffer a huge drop in fuel economy when driving in the city.

Acceleration, not speed (4, Insightful)

5pp000 (873881) | about 6 years ago | (#25348721)

In a small, aerodynamic car, speed doesn't matter that much. (In a larger vehicle and especially trucks, with their poor aerodynamics, speeds above 60 do start to affect mileage more strongly.)

But how vigorously you accelerate can make a big difference. In the worst of the gas price spike I made a point of accelerating gently and shifting much earlier than usual, and found my mileage improved by 15%.

Re:Acceleration, not speed (1)

mferrare (65039) | about 6 years ago | (#25348927)

I want to second this.

I did some tests in a Mazda 6. Fuel economy at 55-60km/h is quite good - around 5-6 litres/100km (pardon all the metrics you US guys). But accelerating up to 55km/h the economy goes down to 20-35l/100km. Even worse if I put my foot down. So in city driving (ie: stop-start), the more gently you accelerate the better your economy. I guess what follows is that the slower you drive the better too simply coz you don't need to accelerate for so long to get to your final speed.

Trading fuel for speed (1)

gmuslera (3436) | about 6 years ago | (#25348729)

In bottom side of the scale, you can walk, is slower than 30mph, but in reasonably short distances is not that bad, and is healthy even. Or drive a bycicle, that is faster at least. Public transportation could be another cheaper alternative, if fits.

Only use a car/gas just when you need what comes with a car (and if what you need includes speed, then will be no slow driving saving anyway)

What works: (-1)

no-body (127863) | about 6 years ago | (#25348733)

- increase tire pressure by 2 psi
- accelerate very slow
- drive with lowest rpm possible and shift up immediately, at 1000/1500 rpm
- turn engine off at red lights - after 10 sec off it's beneficial
- downhill coast and turn engine off (you gotta know what you are doing - PS fails and break booster as well after a while)

brings something like 10 - 20 % increase in city, on Hwy going 45 more.

Postet this before and got heat for the turning off down-hill. I am doing it where possible and I feel like - you can do what you want, not my business.

Re:What works: (2, Insightful)

ArtemaOne (1300025) | about 6 years ago | (#25348751)

Slow acceleration is worse than (reasonably) rapid acceleration. The rest of your tips I can't argue against, but I do know that decent acceleration outside of the red zone is better.

Re:What works: (1, Informative)

base2_celtic (56328) | about 6 years ago | (#25348887)

It's counter-intuitive, but relatively rapid acceleration is far better than slow acceleration.

A long, slow acceleration up to a target speed will use more fuel than a rapid acceleration up to a target speed. This is regardless of the vehicle's horsepower or torque.

Downhill coasting I've covered in other posts in this thread. Suffice it to say that brakes are a safety feature that you don't to have missing in an emergency.

Re:What works: (1)

Wonko the Sane (25252) | about 6 years ago | (#25349019)

Suffice it to say that brakes are a safety feature that you don't to have missing in an emergency.

Unless you're in Oklahoma, in which case you use brakes only for emergencies and also reflexively at the top of every fucking hill and during any curve (no matter how slight)...

Not that simple (2, Informative)

clarkkent09 (1104833) | about 6 years ago | (#25348983)

Postet this before and got heat for the turning off down-hill. I am doing it where possible and I feel like - you can do what you want, not my business.

There is no question that coasting with engine off uses less fuel (zero) but there are other legitimate questions:

Is it necessary? - in many modern cars the fuel is cut off while engine breaking, so in that case you are using zero or thereabouts fuel anyway. On the other hand any savings on fuel (if any) can be easily offset by the extra wear on the brakes. You could also cause major damage to your transmission if you shift into a low gear or even into reverse by accident while moving at high speed.

Is it safe? - No. It's a bad idea to drive in neutral as you cannot quickly accelerate should the need arise, and also increased strain on the brakes makes it more likely that they will fail at the wrong moment.

Is it legal? - Depends where you live, in California and probably some other states it is actually illegal - look up California Vehicle Code 21710

Re:What works: (1)

corsec67 (627446) | about 6 years ago | (#25349053)

- downhill coast and turn engine off (you gotta know what you are doing - PS fails and break booster as well after a while)

And while you are at it, tune your carburetor.

Modern ECUs already turn off the engine when you are coasting down a hill. Fuel injectors don't have to inject something every time the intake valve opens.

I had a Subaru that wouldn't start putting fuel into the engine quick enough, so every time it the engine was really cold and I pushed in the clutch after going down a hill, it would stall.

Re:What works: (1)

Snowblindeye (1085701) | about 6 years ago | (#25349087)

downhill coast and turn engine off (you gotta know what you are doing - PS fails and break booster as well after a while)

I think in older cars that might work. In newer cars its not a good idea because:

a) they already cut of fuel intake when coasting

b) when you start it back up like that, unburned fuel can get into the catalytic converter and seriously damage it

Holden Rodeo experience. (1)

Hucko (998827) | about 6 years ago | (#25348793)

I personally have been using the inertia of the car rather than selecting a particular speed as the 'efficiency speed'. I've gone from 9km/l to 12km/l. I've added 200 km to my 55 litre tank around town by driving up to the speed that will get me to the next point of change without a loss of noticeable time spent driving. I originally based my idea on the Toyota Prius which I had driven for ~ 12 months as a taxi driver. I deliberately practice consideration i.e. I don't do it if it is against the general flow of traffic or drivers are unable to get round me. Gordonjcp has it right, and practice consideration that other drivers may not understand what you are doing so get out of their way.

oops Re:Holden Rodeo experience. (1)

Hucko (998827) | about 6 years ago | (#25348859)

Gordonjcp [] is wrong about efficiency. It does make a difference to fuel economy, at least in my diesel ute. Just as turning off your engine if you are likely to be at a stop light for >10 - 20 s does save fuel. I've been recording the numbers for over 6 months and it has increased efficiency. I haven't had a chance to do it in a small fuel efficient car (barring the Prius of course), but it definitely works in the Rodeo.

scangauge (2, Informative)

Anonymous Coward | about 6 years ago | (#25348797)

Buy a Scangauge 2. Google it. It costs around $150. Hook it up to your car, and it will show you shitloads of fuel economy information and more.

Slower driving (1)

Psychotic_Wrath (693928) | about 6 years ago | (#25348799)

Driving slower can help improve your gas mileage when there are a lot of stops. For example if you drive 10 blocks and you get up to 45mph and have to stop you will use more gas than if you only got up to 30mph and then stopped.

Overdrive (1)

dr_strang (32799) | about 6 years ago | (#25348803)

A lot of people haul out the old 55mph chestnut when economy and fuel efficiency is brought into question, and the simple fact is that the 55 mph law was put into effect when very few cars had overdrives or auto transmissions with more than 3 forward speeds. Since the law was repealed, cars, engines and transmissions have been designed and improved to work optimally at higher speeds.

Re:Overdrive (1)

rwade (131726) | about 6 years ago | (#25348873)

This is a good point. Where are the studies suggesting which speed is most efficient? Is this not something that the EPA can add to its fuel economy tests and post on a model-by-model basis?

Resistance (1, Informative)

Colin Smith (2679) | about 6 years ago | (#25348805)

Air resistance.
Tyre rolling resistance.

Then approximately in order you have;
Air conditioning off.
Engine RPMs constant and at the peak of the torque curve if you can.
Clean your air filter.
Fit iridium spark plugs.
Use a fuel with a cleaning agent every 6 months or so.

And probably illegal;
Chip your car so it runs at the ideal gas/air mixture, not simply one which will allow it to pass the regulations.
Get rid of your catalytic converter.


Re:Resistance (1)

PachmanP (881352) | about 6 years ago | (#25349067)

Chip your car so it runs at the ideal gas/air mixture, not simply one which will allow it to pass the regulations.

um I would strongly suspect that the default is pretty much the "ideal" fuel to air mixture. Car manufacturers spend alot of time and money getting the trade off between fuel economy and not-melting-the-fucking-engine (neglecting of course performance cars where it's power and not-melting...). The most fuel efficient f/a ratio is the stoichiometric ratio where all of the fuel reacts with oxygen. When this occurs "shit gets hot" so they dump extra fuel into the engine to actually cool the exhaust.

That said the offtopic mod for the parent is a little off.

The best way to increase gas milage (2, Funny)

YokoZar (1232202) | about 6 years ago | (#25348855)

Related: are there any practical hypermiling techniques that you've found for people not ready to purchase a new car, nor give up driving generally?

Drive downhill.

And then what? (1)

rwade (131726) | about 6 years ago | (#25348903)

Walk back up?

Just never go back?

jack rabbit starts (5, Funny)

Ydna (32354) | about 6 years ago | (#25348867)

I'd say the way people blast off from the green light like their in a Formula 1 Grand Prix* is probably doing a bigger number on fuel economy in city driving more than anything else.

* or not if you were Hamilton yesterday.

Good luck try to go 55 much less slower on I-294 (1)

Joe The Dragon (967727) | about 6 years ago | (#25348879)

Good luck trying to go 55 much less slower on I-294. The people who go slow like that are the same people who wasted gas by idling and holding up traffic.

Cruise Control? (2, Informative)

Ieatsyou (1383005) | about 6 years ago | (#25348901)

If someone has already said cruise control, I apologize for restating it, but for me it works. The computer in most modern cars can control the speed of the car within a 20th of a mile. (1/20) That is a lot closer than any human can handle. This prevents you from over/under speeding (I.E. You wanting to go 70 MPH but you waiver between 68 and 72). There are other obvious things you can do though: regular car maintenance being the biggest of them all

Use bricks (1)

rwade (131726) | about 6 years ago | (#25348919)

You can also try bricks to hold the accelerator pedal. I've found that mileage may vary, however. After all, you would need a 60 mile per hour brick, 70 mile per hour brick, etc.

Offtopic (1, Offtopic)

banffbug (1323109) | about 6 years ago | (#25348935)

Ride a bike.

I've been car-less all my life. We rent a car when we need one. ~$100/month spent on a car. Live urban, not suburban, and this will be easy.

I drive a Ford Focus (1)

bensafrickingenius (828123) | about 6 years ago | (#25348961)

with a manual transmission. I got it new (picked it up on 9-11-2001 -- felt like a schmuck picking it up that day, but hey, I needed it!). For years I drove as fast as I liked (almost always 10 mph over whatever the speed limit was). For 6 years I averaged 34 to 36 mpg. Then this spring I passed a guy in a Saturn. He was doing 55 mph in a 70 mph zone. He had a sign in his back window that said something like "I'm getting 40 mpg driving at this speed -- please just smile and go around." I gave it a try. I had a long trip scheduled for the next week, and (honest) I got 45 mpg. I've been doing 55 on the highway ever since, and saving a lot of money. My wife is embarrassed to ride with me. She'll get over it.

more time stuck in traffic (4, Insightful)

hazem (472289) | about 6 years ago | (#25348967)

This is just another case where people don't realize (or care) that trying to maximize the performance of one part of the system (their commute) ends up diminishing the performance of the overall system.

Only a few people doing this slow driving will result in large numbers of other driver stuck waiting at more lights. Even worse, this kind of slow driving will result in some other drivers driving recklessly trying to get around the slow drivers. It won't take many crashes, injuries, and deaths to completely wipe out any savings made to the economy by a few people driving slowly (if only from traffic backups due to crashes).

Using these kinds of hypermiling techniques are just fine for an individual who doesn't have any regard for how their behavior impacts others.

Most of the comments here seem to assume highway (1)

dr2chase (653338) | about 6 years ago | (#25348975)

Most of my miles are non-highway, over 1/3 of them are on a bicycle.

Non-highway, the name of the game is to figure out if you're likely to be slowing down ahead, and if so, immediately take your foot off the gas. The only time accelerating pays off much is if you make a light that you would otherwise miss. I have not yet done the turn-the-engine-off at stop-lights experiment, but my understanding is that if you are stopped for more than a few seconds, you win. However, since my starter might not have been designed for that sort of use....

I'm little surprised to read of higher efficiency at much higher speeds, because wind resistance is a bear. To shave 10% off my bicycle commuting time, I must put out 30% more power (but for 10% less time, consuming 20% more energy). When the power is coming out of your own hide, you do notice, and don't need some silly magazine to tell you that you're working harder. The difference between 55mph and 80mph is a factor of TWO in the energy expended -- is the engine really designed to be that much more efficient at the 80mph RPMs?

The big cost is braking (1)

Animats (122034) | about 6 years ago | (#25349001)

The major key to fuel economy is not to brake much. That's when you're throwing energy away. Unless you have something like a Prius, with regenerative braking. Pay attention to traffic lights ahead of you, and coast up to red lights without braking if possible. Pay attention to traffic light synchronization. Many lights are synchronized, at least for a few adjacent lights, for a specific speed. Often, you can drive through a whole set of lights at a fixed speed without stopping. Allow extra space ahead of you and use it to avoid as much braking as possible.

For an amusing example of this, see the Great Highway in San Francisco, which runs for several miles along the beach with no place to turn off, but frequent traffic lights at pedestrian crossings. The traffic lights are all synchronized, and if you drive exactly at the speed limit, you should never have to stop. But watch driver behavior there. Even in this ideal case, people are speeding up and slowing down.

For internal combustion engines at low speeds, heat loss, the "idling cost", within the engine dominates. This is not true of electric motors, which have no idling cost This is why electrics do so well at low speeds.

At a fixed speed, rolling resistance and air resistance dominate. Tire pressure matters far more at higher speeds. Air resistance goes up with the square of the speed; below 30MPH or so, it barely matters; at Formula I speeds, it dominates everything. (Formula I racing cars are using maybe half their energy in the airfoils used to push the car down into the road to get more traction. However, a spoiler on a passenger car is a lose below 90MPH or so.)

It is engine/car specific (really RPM)... (1)

Fallen Kell (165468) | about 6 years ago | (#25349007)

It is all about the RPMs that the engine is doing. The more RPMs, the more times fuel is injected into the engine and burned per minute. It is really that simple. If your engine has a good gear ratio, maintaining speed at 2,000 RPMs at 50mph will be better fuel efficiency then 45mph at 3,000 RPM in the previous gear. However, that said, a 4 speed automatic won't usually be able to shift into a higher gear at a lower RPM than what is set in the computer to begin with, which is why manuals get better fuel economy than automatics (well that and the fact that the "manual" can shift gears ahead of changing conditions since the control mechanism can usually see a 50-100 feet ahead of the vehicle, whereas an automatic can only see the road that thing it is currently at).

There are plenty of other ways to save fuel (1)

hacker (14635) | about 6 years ago | (#25349009)

I realize that "slowing down" may seem like an obvious tactic to some (and maddeningly annoying to others), but there are plenty of other ways [] to save fuel costs... improper tire pressure, improper alignment, excess weight in the trunk and others are documented and explained there (I count 13 other things in the article linked here).

Give it a read and pass it on, you might help others conserve as well.

Engine Efficiency, and a ton of factors. (1)

thesupraman (179040) | about 6 years ago | (#25349011)

There are a lot of factors.

Drag does up with speed, and is non-linear and vehicle dependent (a low drag car tends to also increase slower and later)

Other losses (drivetrain, tyres etc) tend to increate, some with RPM, most with speed.

Road surface makes a difference!

Engine efficiency is the complex one, engines are very inefficient at high AND low speeds.
for a first-order approximation, your engine is probably (depending on design) most efficient at the lower end of its torque peak.

However, the #1 difference is driving style.

The most efficient styles of driving can be VERY surprising - accellerating at the torque peak until a speed above your target and then coasting slower is one of the best, but a real pain to do.
Even for normal driving styles, a variance of 20% is not uncommon, especially in city driving.

The upshot is that slow driving is not by any means the answer to fuel efficiency.
Fast driving is even worse though ;)

#1 rule of fuel efficiency, learn how to drive your car, and get a realtime fuel consumption monitor, log each tank of gas, and learn!

Let 'em think it (1)

ohxten (1248800) | about 6 years ago | (#25349013)

Let them think driving 30 in a 45 zone will give them better mileage; maybe we'll have a heckuva lot fewer deaths.

Or, people will still drive just as stupid and it won't have an effect.

Just use common sense... (1)

Eric Damron (553630) | about 6 years ago | (#25349027)

I just purchased a Prius this year. Not with the idea of saving money on fuel (I'll never save enough to make up for the cost of the vehicle.) but for environmental reasons. However, having a car that gives me constant feed back about what mileage I'm getting has made me change the way I drive.

I do drive more slowly. Not because my mileage goes up from the lower speed but because I like more distance between my vehicle and the vehicle ahead of me. It allows me to take my foot off of the gas and coast when the vehicle ahead brakes. If I can slow down by coasting rather than wasting my momentum by braking I'm saving gas. Many times the car/truck ahead will brake in order turn onto a side street. If I don't have enough distance I have to brake myself. I try to anticipate and slow down on my terms.

Also I don't stomp on the gas peddle to accelerate. Gas mileage is horrid during acceleration even in a Prius.

In the US the Prius doesn't come with a EV mode. Although the computer is aware of that ability, US customers are not given a way to engage EV mode as they are in Europe. There are many hacks that allow a person to access this mode and I have installed one but find that the short range (Less than two miles.) makes it almost useless. Even the next generation of the Prius that is suppose to be pluggable is only suppose to have an EV range of about ten miles. Still pretty lame.

not about the gas (1)

blindbat (189141) | about 6 years ago | (#25349029)

There have always been people that drive much slower than the posted speed.

I'm not sure if they are stupid or lost. But they do drive me crazy.

I don't think it has anything to do with saving gas.

Hybrids (1)

rm999 (775449) | about 6 years ago | (#25349039)

Hybrids are becoming increasingly popular, especially amongst those who consider fuel consumption a priority. I just purchased an Altima Hybrid a few months ago, and I have found (circumstantially) that most of my gas savings come from lower average speeds. Driving above 40 mph or accelerating quickly forces the engine to be on, which cancels most benefits of a hybrid (in fact, the extra weight of the electric motor and battery can actually reduce mileage compared to a non-hybrid.)

Don't be aggressive (2, Interesting)

hansamurai (907719) | about 6 years ago | (#25349045)

The best hypermiling technique I've found that anyone can do is don't be aggressive on the road. This is pretty obvious but I used to drive like a jerk and weave in and out of cars, constant slamming on breaks and jamming the accelerator. Then gas hit $2.50 and I had a baby on the way so I dramatically changed my driving habits. I coast a ton and never tailgate (well, I do draft behind semis sometimes on the highway). My MPG has gone up a ton and I was basically paying the same at $2.00 and $3.00/gallon for a tank of gas. I do mostly city driving so it's tougher to keep a constant 55 MPH (seems to be my optimum speed), but I just don't drag race from light to light anymore.

What I'm trying... (1)

FlyByPC (841016) | about 6 years ago | (#25349071)

I commute home from work late at night a lot (and head to work mid-day), so I get the opportunity to drive using the brakes as little as possible. The idea is to think ahead and figure out where you need to be to have the best chance of not having to slow down suddenly. I think of it as if I were having to pedal the car -- would I really want to speed up here, knowing I'm just going to have to waste that kinetic energy by braking?

I usually manage the first fifteen miles or so without touching the brakes once on the main road -- there's no acceptable way to slow down enough for the toll bridge without the brakes.

Other things I've tried:
  • Driving a bit more slowly (for me, this means staying within 1-2mph of the limit in most cases)
  • Making sure my tires are correctly inflated (you'd be surprised how low tires can be and still look okay)
  • Getting everything ready to go before starting the car (seatbelt, mirrors, etc)
  • Using the AC as little as possible (within reason) and leaving the windows up at highway speeds. I consider the vent/fan to be free.
  • Keeping the junk that I usually carry around out of the car. Yeah, it's useful to have a gallon of antifreeze, jumper cables, a battery charger etc -- but realistically, I'm never more than a few hundred meters from help, especially with a cell phone.

I don't have any hard data on how much this is helping -- but I do seem to go farther on a tank of gas than I used to. I generally get about 30mpg combined. ('97 Escort wagon w/automatic)

Having experimented around with it a little... (1)

subreality (157447) | about 6 years ago | (#25349075)

There are simple driving habits you can do to save some gas.

#1, anticipate when you have to stop. If you're cruising along at 50mph on a country road, and you know there's a stop sign up ahead, get off the gas early, and coast down, so you start applying your brakes at 25mph instead of going straight from cruise to braking. Traffic lights are harder, but if you can anticipate the timing, you'll coast up to the light as it's turning green, instead of going full power up to it and then braking to a stop, and then having to expend all that energy to get moving again. Similarly, slow down early for curves - just coast down, instead of having to use your brakes. Regenerative braking stores and reuses this energy in these situations - without it, you have to not use the energy in the first place.

#2, work with hills, not against them. When going up a hill, don't downshift. Use a max of 75% of your engine's power in high gear, and accept that it's OK to lose a little speed. Reach the top at 35mph, go down the hill and pick your speed back up to 45, and then burn it off again going up the next hill. This lets you keep your engine at efficient lower revs, and reduces energy wasted in braking. It's easier with a stick shift, but in an automatic you can learn how far you can push it without downshifting.

#3, don't bounce the throttle. A lot of people constantly move back and forth between 0% and 50% throttle to regulate their speed, instead of just pushing to 25% and holding there, and letting their speed wander 1 or 2 mph up or down, or gently adjusting the throttle by 5%. Your engine's computer fine-tunes fuel-air ratios best when it's running steady state; when you make lots of fast changes, it has to make lots of guesses about fuel flow, and it errs on the side of too rich (preventing damage to the engine at the expense of wasting gas). This is why cruise control tends to save gas (it makes slow, smooth throttle adjustments), but I've found I'm easily able to do better in many situations, especially combined with #1 and #2 above.

#4, understand your MPG meter. Some people are bothered that they get single-digit gas mileage when they're accelerating, so they accelerate REALLY SLOW to make their mpg always stay above 15 or so. 15 MPG for 30 seconds is actually worse than 5 MPG for 10 seconds (you spend more time in lower gears), and you're screwing up traffic flow when you do it, making everyone behind you waste gas too. Step into it (say, 75% throttle, shifting at 60% of redline in a normal family car), accelerate up to speed quickly but smoothly, and then get into high gear and start reaping your cruise mileage sooner.

In all of these techniques, pay attention to your effect on traffic around you. Big gains can be had with relatively subtle changes. If you're varying your speed around too much, you're obnoxious, dangerous, and wasting everyone else's gas trying to get around you, so stay aware of what you're doing to others.

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