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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.

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closed as off topic by NoCarrier Apr 20 '11 at 5:12

Questions on Motor Vehicle Maintenance & Repair Stack Exchange are expected to relate to motor vehicle maintenance or repair within the scope defined by the community. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about reopening questions here.If this question can be reworded to fit the rules in the help center, please edit the question.

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@NoCarrier: I disagree with the closing of this question. I understand you don't want driving questions, but 1) Its about what is going on under the hood and the maintenance consequences different uses of the car 2) It is a question that I need to pose to an expert and requires a "deep, highly technical conversation". –  yhw42 Apr 20 '11 at 15:15
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I agree with yhw42. This is a question that deepens one's understanding of the inner-workings of a car; that's something that any diy auto enthusiast would be interested in. It may be posed as part of a driving question, but in the end I think it's useful. –  Annath Apr 20 '11 at 18:09
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@NoCarrier, I think this question could be rewritten to make it more appropriate to the main thrust of the site. For example, changing the title to something like "Will engine braking damage my transmission? How can I tell?" makes it less of a driving question and more of a mechanical one. –  Bob Cross Apr 21 '11 at 13:57
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3 Answers

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.

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@yhw42 my understanding is also that it is safer to coast with the transmission engaged. If the engine dies for some reason while coasting and the transmission is NOT engaged then you lose the hydraulic braking power but if it is engaged then even if the car were to die, the momentum keeps the engine turning providing the power for the hydraulics –  Patrick Apr 13 '11 at 0:20
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@Patrick, the brake booster is typically powered by engine vacuum. I am not sure whether a stalled engine would continue to provide vacuum, even if it were being forced to spin by an engaged transmission/clutch. Regardless, even with the engine off, the brake booster vacuum reservoir would provide assist for one or two pedal applications even after the engine stalled. –  William Cline Apr 13 '11 at 1:59
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A stalled engine would still provide vacuum under this scenario, because new air is still being pulled into the engine. The fact that the energy for the intake stroke is coming from the car's momentum and not the firing of another cylinder won't matter. –  Ukko Apr 19 '11 at 21:34
    
This is a really great thought-provoking question and answer. It kind of helps explain why braking in an electric car puts more energy back into the system. –  jmort253 Apr 20 '11 at 3:17
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@Bob - Actually, it's important to determine the boundaries now, not once the rifraff has taken over your Q&A site. It's much easier to keep the bad stuff out now than it is to try to get rid of it later. Not meeting the number of questions is a short term goal. Focus on the long term and the site will be fine. With that said, I do agree with you that this question should be reopened, and I'll vote too. It's like that restaurant you went to that had a roach crawl across your plate. It's been ruined for you, and nothing that they can do will ever fix it. Online communities are the same. –  jmort253 Apr 22 '11 at 7:03
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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.

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On the pedal down vs. pedal up point. Holding the clutch down does not wear on the clutch but rather the throw-out bearing, usually when you replace a clutch you also replace this bearing. In the old days these bearings were very hokey and it was wise to limit the wear on them--today, not so much. With the exception of putting a brick on the clutch and running the engine for days to prove me wrong, I would expect the clutch to wear out first from normal use. Of course YMMV. –  Ukko Apr 19 '11 at 21:48
    
Thanks for the correction, @Ukko. I've edited my answer. –  William Cline Apr 20 '11 at 1:23
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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.
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