Coasting down mountain roads

I live in western Washington (the state, not DC), and we have some pretty long hills when going over the pass. I used to have a standard transmission, and when I’d have a long downhill stretch on the interstate, I’d put it in neutral and coast until I’d get to the bottom, rev the engine to about 2500 rpm, and put it into gear. No problem. Now I drive an automatic, will I hurt the transmission if I put it in neutral, than back into gear when I get to the bottom of the hill? Usually speeds will be in excess of 70 mph.

Why do you want to? The engine shuts off fuel when you’re going down hill because the wheels keep the engine turning. If you put it in neutral, the engine can’t shut off fuel, so you actually waste gas.

No, you won 't hurt a modern transmission. One of the added expenses incurred in cars in this day and age, is to make cars more impervious to damage caused by drivers doing things to them not recommended in the owners manual.

I had a rental car, Chrysler 300, I set cruise control on and it downshifted as needed to maintain desired speed.

This has been discussed ad nauseam in past threads. There is absolutely no reason to put any car in neutral to go down a mountain. Leave it in “D”, and downshift to the next lower one if you have to use your brakes too much. Shadowfax covers it above, too.

Putting it in neutral has zero benefit. And, assuming you’re driving a late model vehicle (you didn;t tell us what you’re driving), it’ll do no harm.

Thanks guys. I drive an '05 Titan, love the truck, but not the gas mileage, wish it were better. Good to know the fuel is shut off on downhills, I’ll keep it in D from now on.

Good man. Stay out of neutral no matter what kind of trans.

However, depending on how steep the grade is you may actually may want to downshift (D3, D2, or whatever your truck has) to use the engine to slow down. This saves your brakes and prevents them from overheating. You don’t want to test those runaway truck ramps, do you?

Sometime in the last year when this discussion arose, a self-described hyper-miler contributor claimed he got better gas mileage putting a car in neutral when going down hills as opposed to leaving it in gear.

His point was even though leaving the car in gear would allow the injectors to shut off, he found that in neutral he could coast lots farther, which more than offset the fuel used while idling during coasting.

I’m very much against coasting in neutral and agree with all the previous replies to this thread.

It is clear that for cars left in gear, the recent technology to shut off injectors allows a car to use less fuel than a car that keeps its injectors working. Works as designed - great invention.

Where I’m less convinced is this:
Is it possible to get better mileage by coasting (farther) down a hill in neutral with the engine idling, than it is to leave the car in gear (with the injectors shut off but incurring engine drag and less coasting distance) ?

Joe, that has some merit, but a lot of self-styled hyper-milers have absolutely no consideration for road safety. They’ll blow through stop signs, take corners at 45mph, drive 30mph on the interstate, etc, all in the name of fuel economy. Frankly, those guys should be jailed.

So, if you start at the top of the hill and coast all the way down in neutral and then leave it in neutral as you glide through the city, skidding around corners on 2 wheels until you’re finally doing 10mph on a boulevard before you finally decide to put it in gear, you might save about an ounce of gas.

But if you hit me while you’re doing it, my lawyer’s gonna kick your butt.

Good response “shadow”.

I think it’s possible, with just the right downward slope, to save gas in neutral.
It has to be where you’d need to use a little throttle to make up for engine braking, yet could maintain steady or slightly increasing speed in neutral.
It’s a narrow window, not worth the risk IMHO.

That “narrow window” becomes a lot wider if you’ll settle for plus or minus a few mph of your intended cruising speed instead of insisting on going that speed exactly.

I was trying to sound conservative and avoid being scolded by the ‘never ever coast’ folks.

I believe at one time it was a major offense to coast down a hill in neutral. I cannot comment on current laws, but would be surprised if such laws were repealed.