Does tire size impact your speedometer readings?

Dear Car Talk

Dear Car Talk | May 01, 1999

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

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."

Wait! Before you buy your next car, make sure you read Tom and Ray's guide How to
Buy a Great Used Car: Things Detroit and Tokyo Don?-t Want You to Know. Send $3
and a stamped (55 cents), self-addressed, No. 10 envelope to Used Car, PO Box
6420, Riverton, NJ 08077-6420.

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