About the Tire Size Calculator
The Tire Size Calculator computes the full dimensions of a tire from its marking and shows the results in metric and imperial units. Enter a size like 225/45R17 and you get sidewall height, overall diameter, circumference, and revolutions per mile. It also compares two tire sizes and reports the speedometer error between them.
The tool uses the standard metric tire marking method from the ETRTO standard: sidewall height equals section width multiplied by the aspect ratio, and overall diameter equals rim diameter plus twice the sidewall height. You enter width in millimeters, aspect ratio as a percentage, and rim diameter in inches. For a comparison, enter a second size and a driving speed. Pick a replacement with a diameter close to the original, because larger deviations increase speedometer error.
- Enter the width, aspect ratio, and rim diameter from your current tire sidewall.
- Add the marking of the replacement tire you want to compare.
- Read the diameter difference and speedometer error before you buy.