How accurate is cruise control?

Dear Car Talk

Dear Car Talk | Jan 01, 2001

Dear Tom and Ray:

How accurate is cruise control? I'm a state trooper, and I catch a number of people speeding who say that they couldn't have been going "that fast" because they had their cruise control set at ... . How accurate are these things? -- Benny

RAY: Hey, you're a thoughtful guy, Benny. I use that line all the time, and the troopers who stop me never even pause before writing out a ticket.

TOM: We've given your question considerable thought, Benny, and our guess would be that the maximum error in the entire cruise-control system -- all factors considered -- could be as much as 10 percent. And that includes "temporary" factors like hills and wind that momentarily throw off the speed of the car, and "permanent" factors like oversized wheels and tires that affect the speedometer reading.

RAY: And if all of those factors conspire against a driver at a given moment, I would say it's possible -- possible -- for the cruise-control setting and the speed of the car to be off by as much as 10 percent, or about 6.5 mph at 65 mph.

TOM: So if a guy's got the wrong-size wheels and tires on his car and is hit with a tail wind while going downhill just as he's passing your radar gun, and you pull him over doing 71, and he says, "Gee, officer, I had the cruise control set on 65," he might be telling the truth.

RAY: But our calculation fails to take into account the single most important variable: the D.E.F. (Driver Exaggeration Factor).

TOM: And I would guess that the average D.E.F. is about 9 mph. So when you factor that into the equation, you can conclude that if someone claims he had the cruise control set at 65, he was probably going at least 74. Hope that helps, Benny.

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