I Changed My Tires; Was I Too Speedy?

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Mounting different-sized tires than the ones that originally came with a vehicle can have an effect on the accuracy of the speedometer and might eventually lead to a speeding ticket. However, many people like to install bigger tires than the ones originally on their vehicles.


This Demonstration gives the actual speed of the car as a function of the diameters of the original and new tires and the speedometer reading.

The formula applied in the Demonstration is: actual speed = (speedometer reading) × (new tire diameter) ÷ original tire diameter

When measuring the diameter of a tire, take into consideration that when a tire is on a car, the bottom of the tire is flattened due to the weight of the car, and its diameter will be slightly smaller than it was unmounted.


Contributed by: Margarita Ryan (May 2015)
After work by: The Charles A. Dana Center

Open content licensed under CC BY-NC-SA




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