Can cruise control help control your speed going downhill?

Dear Car Talk

Dear Car Talk | Feb 01, 2002

Dear Tom and Ray:

My father-in-law owns a 2001 Ford Taurus SE. He has owned several Tauruses and Sables in the past 10 years, and he insists that the cruise control will hold his speed down when descending a hill. I own a 2000 Taurus and told him that he is full of it. Even after at least one speeding ticket, he still insists that I don't know what I'm talking about and that "at least it USED to hold the speed down." Who's right? -- Dave

TOM: Dave, it's not good for family relations to humiliate your father-in-law. So we'll trust that you'll apply the answer we're about to give you with all due sensitivity, tact and kindness, OK?

RAY: The old goat is nuts, Dave. The cruise-control system operates the throttle only.

TOM: Now, having said that, I can tell you why he thinks it's braking for him. Let's say you're driving down a long grade, on a highway, for instance. Normally, if you start to go too fast, you back off the gas pedal. But you rarely back all the way off, because that would be jarring. Instead, you back off a little bit and "accelerate less."

RAY: But the cruise control can back all the way off the gas pedal if you exceed the set speed by more than a few miles per hour. And when you let off the gas pedal entirely, you do experience the natural braking action of the engine, which results from the friction of the moving parts and the pistons having to compress air in the cylinders. And that's probably what he's feeling.

TOM: So if you have a kind bone in your body, Dave, explain to the old man that what he's feeling is the natural braking action of the engine when it's not accelerating. Tell him it makes perfect sense that he would experience this as braking, and that heright, he can feel a "braking action" of sorts associated with the cruise control.

RAY: And try not to end the explanation by jumping up and down on his sofa and yelling "naah nah nah naaah nah!"

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