Tire Size Comparison Calculator

By AutomotiveCalc · Full tool, formula & sources

Assumptions and review status

Assumes calibration matches the original tire and is not changed. This is an illustrative geometry comparison, not approval of a size change. Do not infer safe fitment from a percentage threshold.

Automated checks do not replace independent professional review, which has not taken place.

Your numbers

Example calculation

Example results are shown below. Controls become available when the calculator loads.

Choose mph or km/h for the speedometer estimate. Changing the unit reinterprets the speed number; enter your indicated speed in the selected unit.

Have a size code? Apply it here, or edit the dimensions below. Supports R or ZR metric sizes, including 225/45R17 91W and 225/45ZR17 (94Y) XL. Only dimensions are applied; load, speed and XL/RF markings are not evaluated. Size codes do not confirm vehicle fitment. How to read tire sizes →

Diameter difference1.42 %
Estimated actual speed60.85 mph
Original diameter24.97 in
New diameter25.33 in
Nominal tire diameter comparisonOriginal diameter 24.97 inches. New diameter 25.33 inches. Both circles use the same scale. Inner circles show wheel diameter.OriginalNew
Unloaded geometry at the same scale. Dashed circles show the wheels. Actual tire dimensions can differ.
Dimensions compared
MeasurementOriginalNew
Diameter (mm)634.30643.30
Sidewall (mm)101.25105.75
Wheel (in)17.0017.00
Speedometer comparison — assuming original calibration was accurate
Indicated (mph)Estimated actual (mph)
30.0030.43
45.0045.64
60.0060.85
70.0070.99

This comparison does not establish safe fitment, clearance, load capacity, or compatibility with vehicle electronics. Use manufacturer-approved specifications.