Fuel Efficiency and Slow Driving?

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 truckandbarter.com 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?

Fuel economy

[JWSmythe (446288)]

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

[JWSmythe (446288)]

    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

[Goldberg's Pants (139800)]

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

[cailith1970 (1325195)]

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

[mustafap (452510)]

>not to mention your spigot bearing.

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

Re:Fuel economy

[cailith1970 (1325195)]

Definitely a mechanic ;) The spigot bearing is the little bearing in the back of the crankshaft that the input shaft of the gearbox fits into to hold it at both ends. When the clutch is not engaged, the crankshaft and input shaft spin together, so the spigot bearing doesn't spin (well, it spins at the same speed as the crankshaft, if you get what I mean). These little bearings aren't really designed for serious long term spinning, so constantly riding the clutch is going to wear it out pretty quickly.

Re:Fuel economy

[JohnnyBGod (1088549)]

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

[rwade (131726)]

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

[base2_celtic (56328)]

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

[aliquis (678370)]

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

Re:Fuel economy

[Sique (173459)]

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

[MentlFlos (7345)]

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.

Wrong.

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

[base2_celtic (56328)]

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.

My plan for improving the world's fuel consumption

[Joce640k (829181)]

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

[Rogerborg (306625)]

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

[base2_celtic (56328)]

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.

Take advantage of aerodynamics

[kbrasee (1379057)]

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

Thanks!

[ciaohound (118419)]

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!

It's about acceleration

[by Anonymous Coward]

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

[Cordath (581672)]

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.

Acceleration, not speed

[5pp000 (873881)]

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%.

jack rabbit starts

[Ydna (32354)]

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.

more time stuck in traffic

[hazem (472289)]

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.

The leanset way to drive...

[i (8254)]

..is accelerating relatively fast to something like 70mph,
then pull of the engine and roll with no gear until You reach
something like 10mph when You start the engine and repeat.

This is the empirically show best method.

But it will probably irritate other drivers...

Re:Its not fuel economy, its self righteousness

[$RANDOMLUSER (804576)]

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.