Downhill or uphill, it won't harm transmission.
Dear Car Talk: I like using cruise control, for a couple of reasons: It makes for more relaxed highway driving, and it improves fuel economy. However, I am concerned that I may be misusing it. Instead of riding or pumping the brakes when descending hills in the mountains, I click the “reduce speed” button in hopes that it will slow the car down. In some cases, it appears to work on long, not-so-steep downgrades. On steeper hills, it doesn’t seem to slow me down. Am I messing up my transmission by using my cruise control to slow the car? — David
A: No, not at all. In traditional cruise-control systems, using the “reduce speed” button is just like backing your foot off the gas pedal. It’s harmless.
Let’s say you’re driving on flat ground and your foot is on the gas pedal, keeping your speed at 65 mph. Then you come to a downhill grade, and you lift your foot halfway off the pedal.
If the grade is steep enough, you’ll gain speed anyway, even without accelerating as heavily. But if the downhill grade is shallow, you very well might slow down. After all, there’s always wind resistance and tire friction trying to slow down your car, and it takes a pretty steep hill to add to your momentum at 65 mph.
Anyway, that’s exactly what you’re doing when you reduce the speed setting on your cruise control. It’s like backing your foot off the gas.
So you’re not doing any harm. If reducing the speed on your cruise control doesn’t slow you down, then you have to either downshift to a lower gear, or use your brakes to keep from gaining too much speed.
There is a new type of cruise control that’s available on more and more cars now, and will be available on all cars at some point. It’s called “adaptive cruise control,” and it does use the brakes to slow you down. It uses some of the early elements of self-driving cars to keep you at a set distance from the car in front of you — even if that car slows down or stops. If you had it set to 65 mph and drove down a steep hill, it would use the brakes, if necessary, to keep you at about 65 mph.
So as soon as you do find a way to ruin your transmission, you can get rid of your current car and buy one with adaptive cruise control, David.
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