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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 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 at 19:56

4 Answers

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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
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 at 22:41
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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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Why do you think tow trucks hook up cars facing backwards? It's so the 2 wheels on the ground are the front ones - If you tow a car with the back wheels on the ground, it can ruin the car/gearbox. In theory you cannot tow a full time 4wd with any wheels on the ground!

My shogun has an option of 2wd, so if I was going to tow it, I'd select 2wd then hook it up to the tow truck with the front wheels on the ground.

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Your answer is correct only as it is related to rear-wheel drive vehicles. Properly it would be “towing with drive wheels off the ground”. Also, horrible grammar. – theUg Feb 10 at 18:05
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Don't do it. Unless you want your transmission to prematurely wear out and have to pay someone too put a new one in. By the way, it doesn't save you fuel by doing that.

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Care to elaborate on why? – Annath May 6 '11 at 1:58
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Most manuals for cars automatic transmissions state that the car should not be towed with all four wheels on the ground for more than 50 miles. I know of several cars that have had to have their gearboxes/torque converters replaced because they were towed too far. Coasting in "neutral" is going to cause the same issue as far as I am aware. – Mauro Aug 9 '11 at 12:35
Wow, show me long shallow grade that is over 50 miles long. I would love to coast down on that. – theUg May 28 '12 at 22:06
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@Mauro The difference in towing and coasting is the engine is still running and that is turning the front pump in the transmission keeping the transmission lubricated. When you tow the vehicle the transmission isn't being lubricated. – Larry♦ Feb 10 at 18:02
Yeah, they are separate situations. Coasting in neutral is not going to hurt anything. – Nick Feb 10 at 22:38

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