Tire Size & Miles Per Hour – Using larger tires on a vehicle changes the miles per hour. Speedometers are set to calculate mileage based on the stock tires that come with the vehicle. If stock tires are replaced with larger models, this will change the miles per hour (mph) and the speedometer reading will no longer be accurate.
Calculating Actual Speed
To calculate actual speed with larger tires, use the following equation: new tire diameter times speedometer-indicated speed divided by old tire diameter equals actual speed in miles per hour. As an example, 36 inches times 55 mph divided by 28 inches equals 70.7 mph.