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When I drove a manual transmission, I got in the habit of shifting to neutral on long, shallow grades to save fuel. Now I'm driving an automatic (specifically, a recent Corolla) and still doing it.

But someone told me recently that being in neutral while moving can cause an automatic transmission to overheat, since the fluid is not being circulated.

Is this correct?

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See: mechanics.stackexchange.com/questions/678/… it really isn't more fuel efficient than engine-braking. – Annath May 6 '11 at 0:47
    
@Annath: If coasting allows you to maintain your speed without pressing the accelerator, while engine braking causes you to loose momentum going down the hill and need to apply the accelerator again, it's much more of a toss-up which conserves fuel better. The best way to conserve fuel is of course to cut the engine completely off and coast in neutral, and let the transmission restart it (so you don't expend power on the stater motor) when you need the engine again. – R.. Feb 10 '13 at 17:00
    
with modern engines leaving the car in gear will require no fuel for going down hill, however, shifting it to neutral will require fuel to keep the engine going so its not as efficient as you may think – Mauro Feb 11 '13 at 19:56
up vote 9 down vote accepted

I don't think it will make a difference on gas mileage either way. Anytime the engine is running the front pump on the transmission is being turned by the torque converter and is lubricating the transmission which is also circulating fluid through the transmission cooler.

I don't see how it could cause premature failure of the transmission. Technically you are shifting out of gear and back into gear one extra time when you decide to do this, but it's not really any different that a regular shift so it shouldn't cause extra wear.

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"I don't think it will make a difference on gas mileage" -- This is a sideline to the main question, but why is that? When the car is in neutral, I can go longer without having to use the gas pedal. To me, that seems like it ought to improve mileage. – Michael Myers May 11 '11 at 15:21
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@Michael Myers, engine braking is more efficient - we discussed this here: mechanics.stackexchange.com/questions/661/… – Bob Cross Aug 6 '11 at 16:03
    
@Bob: Your comment makes no sense. Engine braking is more efficient than not-braking when the goal is to keep moving, rather than to slow down?? I think you misunderstood what OP is trying to achieve, which is keeping enough momentum while driving downhill so as to maintain speed without pressing the accelerator. By the way, you can save A LOT more gas doing this if you cut the engine and just release the clutch in gear to restart it (no need for starter motor) on a manual, but that's potentially unsafe. – R.. Aug 7 '11 at 7:59
    
By the way, I see how engine braking is better when the hill is steep enough to maintain speed (you can keep the engine turning with no fuel), but not on borderline hills where you'll quickly lose speed if you don't either go into neutral of hit the accelerator. – R.. Aug 7 '11 at 8:04
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Sure, in that edge case you can save fuel if you shift to neutral and turn the engine off. Whether the fuel saved outweighs the wear and tear from the engine stopping and starting is probably not a bad question to ask at that point. – Nick Feb 10 '13 at 22:41

Auto transmissions are designed to operate similarly to manual transmissions. If you are coasting with an automatic transmission, all you are doing is the same as when you coast with a manual transmission.

It cannot damage the box if the engine is running and your transmission pump is working. But the downside is: do not try to put it into D when in high speed, because that can burn your clutches gradually by making them slip when they are engaging at high speed.
And that will make a hole in your pocket.

Theoretically it cannot damage the box, but is better to follow the guidelines in your vehicle's manual for your own safety.

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It is fine to coast in an automatic in neutral as long as the engine is running. If you stop the engine completely and coast (as is often the case when being towed), you can damage the transmission.

The reason is that the transmission circulates fluid for lubrication and cooling, and this circulation is powered by the engine. Rotating the drive wheels (coasting or being towed) without the engine running causes transmission parts to be moving without sufficient lubrication or cooling, which will wear them out much more rapidly.

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It all depends on the vehicle in question as to whether it saves fuel, and/or is detrimental to the auto tranny.

As long as the engine is running, it is rotating the torque converter, which in turn is pumping fluid around the auto box. This keeps all the bands, clutches, servos etc lubricated no worries. When going back into gear, if you're not rolling up to a stop sign or red light or similar, it may help to increase the engine revs a little to match the revs the vehicle would be doing at that road speed. This will reduce any shock loading on the internals.

As for saving fuel, it depends on the vehicle, carbie cars, and early injected vehicles, it will definitely save fuel. On later vehicles, with more sophisticated electronics, Your mileage may vary. Some vehicles will cut fuel to all/most injectors in a closed throttle position. Where as at idle, and in neutral, all cylinders are getting fuel to keep the engine running smoothly.

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In modern automatics, you should go downhill in D, not N. The reason is that the ECU/TCU is programmed to detect situations in which the wheels are driving the engine (e.g. when downhill or coasting to a stop) and to shut off the fuel injectors, hence zero fuel consumption. If you switch to N, some fuel has to be used to keep the engine idling.

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While your information is good information, it really doesn't apply to the question at hand. They are wondering about the affects on the transmission if allowed to run in neutral, not about how it affects fuel economy. – Pᴀᴜʟsᴛᴇʀ2 Oct 2 '15 at 19:13

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