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I drive a manual transmission '08 Honda Fit.

On my daily 17 mile commute to work, there are three long hills that I can coast down, each about a half mile long. Going down these hills, I've done two different things:

  • I take it out of gear and let the engine idle around 1000 RPM while I coast down
  • I take my foot off the gas and just let gravity do the work, leaving it in gear at 2500 RPM

What is going on in the transmission and the engine under these two scenarios? Do either cause undue wear? Does one save more fuel than another? I am interested in fuel efficiency but I also want my car to last a long time.

Nick C
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yhw42
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4 Answers4

24

What is going on in the transmission and the engine under these two scenarios?

Coasting: nothing much. The transmission is effectively disengaged (it's more complicated than that but it's a reasonable approximation). The engine is idling - burning fuel to keep itself spinning.

Engine braking: the transmission is engaged and the whole system's net friction (from the wheels, axles, driveshafts, all the way to the moving pistons) is acting as a brake. The engine is running in a vacuum state (throttle is effectively closed) and the motion of the wheels are helping to keep the engine spinning.

Do either cause undue wear?

No. I wouldn't advise engine braking down the hill in a low gear near the redline (as you might run past the rev limiter) but you aren't anywhere near that state.

Does one save more fuel than another?

Yes, engine braking is much more efficient. I talk about why that is in this similar question but suffice to say that the free energy provided to the engine from the turning wheels helps a lot. Put your car in the highest gear available to minimize the drivetrain friction on the way down the hill.

Note: a half-mile hill is not very long. You may have trouble detecting the change in fuel economy. That said, you probably won't have to use the brakes going down the hill with engine braking, so there's a savings on brake pads for you.

I am interested in fuel efficiency but I also want my car to last a long time.

Both scenarios are well within the designed bounds of the car. You're not going to notice a longevity increase with either.

Community
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Bob Cross
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6

Here is what is going on in your scenarios:

Coasting in neutral: The engine is running at idle, the gears in the transmission are disconnected from the drive shafts (the drive shafts being what connect the transmission to the drive wheels, which are the front wheels on your Honda). The engine and transmission are suffering no more wear than they would were the car stationary.

Coasting in gear: Your foot is off the accelerator, so (assuming your Fit has a gasoline engine) the air supply to the engine is nearly cut off and it is burning little fuel. It "wants" to spin at idle speed (approximately 1000 RPM), but because it is still connected to the transmission by the clutch and because the transmission is in gear it is being forced to turn at a speed that matches the current gear ratio and speed of the car. The engine is acting as a brake in this scenario, which puts slightly more wear on it and the transmission than if it were disconnected by pushing in the clutch pedal or putting the transmission in neutral.

Coasting in gear will put slightly more wear on the drivetrain, but not a harmful amount. Indeed, sometimes a driver will deliberately rely on the engine braking effect on a long downhill section of road in order to avoid overheating the brakes. The lower the transmission gear the stronger the engine braking effect and the more wear on the drivetrain.

Assuming you were up to cruising speed at the top of the hill and therefore in a high-ish gear, coasting down the hill in that same gear won't hurt anything. I think the difference in wear will be negligible. Which is more fuel efficient, I'm not sure.

A related point: leaving the clutch disengaged while coasting (that is, keeping the clutch pedal pushed in) will accelerate wear on the clutch throwout bearing (credit: comment by @Ukko below). If a driver is going to coast, I think marginally less wear is suffered by shifting to neutral then re-engaging the clutch (i.e., releasing the clutch pedal). However, the extra wear may be negligible on current cars; see @Ukko's comment to this answer.

William Cline
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Coasting is the same as letting the engine idle and burns up fuel. Letting the car in gear on the other hand, uses no fuel at all because your electronic injection system monitors engine load and knows that in such a situation no power is needed and cuts of fuel immediately, turning your engine to an air pump for all practical purposes.

So letting the car in gear is definitely better because:

  • You don't use any fuel at all
  • You don't use the brakes to keep your speed in check.
Engin Kurutepe
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Regarding fuel efficiency, none of the answers took into account that coasting in neutral will mean a longer time before applying the gas pedal since you don't lose speed due to engine braking. I'm unsure if one is better than the other but i think they'd be about equal unless it's very steep or very long where you need to brake.

Sam K
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