Speedometer Error Calculator

How we calculate

Enter the original tire size first and the replacement size second. The original must be the size for which the speedometer was calibrated. You can enter metric notation such as 225/50R17 or flotation notation such as 33x12.50R17. Press Calculate to see the estimated road speed at four indicated speeds. Two tables separate a speedometer marked in miles per hour from one marked in kilometers per hour.

How we calculate: nominal metric diameter equals rim diameter times 25.4 plus twice the width times the aspect ratio divided by 100. Flotation notation supplies the nominal outside diameter directly. We divide the new diameter by the original diameter, then multiply the indicated speed by that ratio. The percentage shown is the actual speed change relative to the indicated speed. A positive result means the vehicle travels faster than the reading under this model.

For example, changing from a nominal 30-inch tire to a nominal 33-inch tire gives a ratio of 1.10. An indicated 60 mph becomes an estimated 66 mph. That same road speed is about 106.2 km/h. If the instrument instead indicates 60 km/h, the estimate is 66 km/h. These are different starting units, so keep the table headings in view when reading a result or sharing a comparison.

This calculation assumes the original calibration was accurate and that wheel speed is the basis of the reading. It cannot account for an existing instrument bias, electronic recalibration, tire slip, or changes in the loaded rolling radius. Inflation, wear, vehicle load, and the particular tire model also affect measured travel per revolution. A size printed on a sidewall describes nominal geometry rather than a road measurement.

Use the table to understand the direction and approximate scale of a change before discussing it with an installer. It does not establish legal instrument compliance or vehicle clearance. Verify the original size against the vehicle documentation, check the replacement specifications, and follow the vehicle maker's calibration procedure where applicable. The comparison calculator shows the associated diameter and width changes alongside this speed estimate.

Frequently asked questions