Using cruise control on downgrades won’t harm your engine

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

RAY: 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.

A weaving car is not safe to drive

Dear Car Talk:

I have a 1999 Toyota Corolla with 213,000 miles on it. About six months ago, it started to wobble down the interstate. I could not get it to stay straight. I would have to “oversteer” to correct it and keep it in my lane. I got four brand-new tires and had them balanced (I needed tires anyway), then had a front-end alignment, and it still does it. It terrifies me to be on the interstate going 50 or more miles an hour with the car weaving on the road. Can you tell me what is wrong with it? Thanks. – Lucy

RAY: It does sound like a classic alignment problem. But you’ve already had it aligned. So I have to start by scolding you for taking away my best answer!

I think you know it’s not safe to drive, Lucy. But I want to reinforce that. You have to get it fixed before you go back out on the highway again for a leisurely Sunday weave.

If you’re having trouble finding a mechanic who can figure it out, try going to www.mechanicsfiles.com. That’s a database of good mechanics personally recommended by people who read our column and listen to our radio show. You put in your ZIP code, and you get some honest, unbiased recommendations.

One thing I would ask your mechanic to check is the steering coupling. Your steering column goes through the firewall into the engine compartment, and then it has to make a bend, down toward the steering rack. That coupling usually uses two universal joints. And sometimes, on older cars, one or both of them can seize up. That can make the steering feel very odd. It can make it hard to turn the wheels for a second, and then the steering can suddenly break free and get easier, causing you to “oversteer.”

So have your mechanic check that first. If that’s not it, I’d probably check the alignment again, just to be sure the guys who say they aligned the car actually did it, and did it correctly.

But whatever it is, Lucy, you have to get to the bottom of it before you end up at the bottom of something.

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