Speedometer Error Calculator

Compare original and new tire sizes to calculate speedometer percentage error and actual speed.

Original tire

New tire

Indicated speed (mph)

—Actual speed

—Speed difference

—Diameter / speed %

—Actual at 70 indicated

—Pure tire-diameter estimate; vehicle calibration and real rolling radius can differ.

How to use this Speedometer Error Calculator

Estimate actual road speed after changing from one tire diameter to another. Enter your values above and the result is calculated directly in your browser. No account is required.

Fast answer, clear inputs

This tool is built around one job: give the answer quickly without forcing a signup or hiding the result. Related tools are linked nearby when the next calculation or conversion is useful.