Dear Tom and Ray:

I'm having a dispute with my father-in-law, who, in a past life, was a large- vehicle diesel mechanic. Even though I'm a banker with no mechanical training whatsoever, I still think he must be wrong. He swears that the size of the tires you put on a vehicle has an effect on the accuracy of the speedometer. I would think that when a manufacturer builds a vehicle, it has to wire the speedometer to something that actually measures the speed of the car, taking into account the variety of aftermarket tires that people inevitably put on their cars. -- Art

Tom: Sorry, Art. As much as this pains you, we have to inform you that your father-in-law is right. Next time you make a bet with him, stick to subjects like derivatives or interest rates.

Ray: The only thing the speedometer measures is the speed that the drive shafts are turning. And it's calibrated for the original equipment tires that come with the car.

Tom: So if the speedometer is just measuring DRIVE SHAFT speed, you can see how putting bigger or smaller tires on the car can make the speedometer read incorrectly.

Ray: To understand it better, think back to Arte Johnson of television's "Laugh- In," the guy who used to ride the little kid's tricycle across the stage. The equivalent of "drive shaft speed" is how fast Arte's feet are pedaling. So if his legs are turning 50 revolutions a minute, in one minute he can make it roughly across the stage at the NBC studios in "beautiful downtown Burbank."

Tom: But if he were on one of those old-fashioned tricycles with the HUGE front wheels, and his legs (i.e., drive shafts) were still turning 50 times a minute, he'd be out the door and halfway across the parking lot in a minute -- which means he'd be travelling a lot faster.

Tom: Now that's an extreme example. Plus, Arte's getting older now, and he probably doesn't get on a tricycle very often. But based on normal variations in available passenger car tires, your speedometer can be as much as 5 percent to 10 percent off for this reason.

Ray: This doesn't create a mechanical problem. Nothing terrible will happen to your car. But in my vast experience with state troopers, they put very little credence in the line, "But I just got new, bigger tires, officer."

What's the best way to warm up your engine in the morning? Find out by ordering Tom and Ray's pamphlet "Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1999 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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