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Speedometer and Tire Sizes

Q: I recently bought a used pickup. The truck has 265/75R16 tires, which are larger than the original equipped tires of 245/75R16. Can you tell me if these larger tires impact the accuracy of my speedometer or my braking potential?

A: If your vehicle has different sized tires than what the auto manufacturer recommends, you run the risk of having an incorrect speedometer and possible braking problems. The diameter of a 245/75R16 is about 30.46 inches, while the diameter of a 265/75R16 is around 31.64 inches (a 1.18 inch difference). The change in diameter changes the circumference, which in turn changes the revolutions per mile. This difference in circumference will affect your speedometer by about 3.9%. Since the truck was originally equipped and designed for 245/75R16s and now has 265/75R16s, at 60 mph your truck’s speedometer would only read 57.7 mph, increasing the likelihood of getting a speeding ticket. Also, running too large of tires puts additional stress on braking components (e.g., brake calipers, rotors) that were designed to handle smaller tires.