I Changed My Tires; Was I Too Speedy?
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.[more]
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.[less]