Tire Size Comparison Calculator

About this calculator

This tool compares two tire sizes written in the standard section-width/aspect-ratio R rim-diameter format (e.g. 225/45R17) by converting each to an overall diameter in millimeters: sidewall height = width × (aspect ÷ 100), doubled and added to the rim diameter in mm. The percentage difference between the two diameters is then reported as your speedometer error, since a speedometer is calibrated for the original tire's revolutions per mile.

Drivers switching to a different tire or wheel size — going up to a larger rim with a lower-profile tire, or fitting a "plus-size" set — use this to check that the new diameter stays within the ±3% most manufacturers consider safe for ABS, traction control and odometer accuracy. It's also standard practice among off-roaders fitting larger mud or all-terrain tires, who need to know how much their actual speed and mileage will now read low compared to the dash.