Do bigger wheels make the speedometer read faster or slower?

Dear Car Talk

Dear Car Talk | Dec 01, 1995

Dear Tom and Ray:

Does putting bigger wheels on your car make the speedometer read faster or slower? I've got a bet on this.

RAY: The speedometer will read slower, Jim. The speedometer reading is based on how many times the axle turns. With bigger wheels, the axle will turn fewer times to cover the same distance, so the speedometer will think the car is going slower.

TOM: If you don't believe us, put on some larger wheels, bring your car up to the "speed limit" on your speedometer, and drive through your favorite local radar trap (preferably with a smile and a wave). I'm sure the nice police officer will be happy to confirm that you were, indeed, going faster than your speedometer led you to believe. In fact, you'll even get a written confirmation of this should you need it to settle the bet.

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