Typically, a car speedometer measures angular velocity of a rotating tire in rpm and then computes speed (linear velocity) based on a standard tire size specified by the original equipment manufacturer. Thus, if the tires are replaced with tires of a different diameter, the speed displayed on the speedometer will be inaccurate. Soon after putting on snow tires, a police officer recorded a vehicle's speed to be 70mph with radar, however the speedometer was only indicating 65mph. What is the ratio of diameter of the snow tires to diameter of the original tires?

Question

whpalmer4 · Answer

The ratio of the speeds is going to be the ratio of the circumferences of the tires, right?  Given the circumference, how do you find the diameter?