Tire Size Calculator

A tire size like 225/45R17 hides the numbers that actually matter: how tall the tire stands and how far it travels in one turn. Enter the section width, aspect ratio and rim diameter and you get the overall diameter, sidewall height, circumference and revolutions per mile. Enter a second size and it compares the two, showing the difference in inches and percent, the new revolutions per mile, and the true speed when the speedometer reads 60 mph. Fitters usually keep a replacement within three percent of the original diameter, which the comparison flags for you.