Dear Car Talk:
I like to put my car in neutral when coming downhill. I think it will save some gasoline, and it gets us down the hill faster. My wife thinks it could damage the transmission or the clutch. Which one of us is right?
It won't damage the transmission or the clutch. But it sounds like it might damage your marriage, Ted.
Since you mention the clutch, I assume you have a manual transmission. If you shift into neutral while going down a hill, you will go faster, and you will save a small amount of gasoline. The downside of going faster is that you can easily go too fast. If you forgo the natural engine braking that you get with the car in gear, you can find yourself rolling at a very high rate of speed on a long decline.
And when you're going that fast -- especially if you're obsessed with not using the brakes so you can save fuel -- you risk losing control of the vehicle or causing an accident if another car changes lanes -- not expecting some guy to come plowing by at 88 mph in neutral.
And if you do use your brakes on a long decline to control your speed, you can overheat the brakes, causing them to fail. So on long, steep hills, we definitely recommend that you stay in gear and take advantage of the engine braking.
And while you will save fuel, the savings is very minor. Modern transmissions run relatively slowly at high speeds, so the difference in engine RPM between high gear and neutral is not very great. In the big scheme of things, you might be saving a fraction of a mile per gallon. And while you're saving those 14 cents, and white-knuckling it down the grade, your wife is adding up the assets she's going to seek in the divorce settlement. So do whatever you think is best, Ted.