Tire Speedometer Error Calculator

Work out the overall diameter of each size, then divide the new diameter by the old one. That ratio is the correction factor: true speed = indicated speed × ratio. A ratio of 1.03 means the dial reads 3% slow — at an indicated 60 mph you are really doing about 61.8 mph.

Slow. A taller tire covers more ground per revolution, so the wheels turn fewer times for the same road speed. The speedometer counts revolutions and multiplies by the factory figure, so it under-reports — you are going faster than the dial shows, and the odometer under-counts distance.

Under about 1% difference in overall diameter the speedometer error is negligible and within the tolerance the gauge is already allowed. Between 1% and 3% it is noticeable but usually fine for a temporary or seasonal set. Past 3% most people recalibrate — some regions require the speedometer to never read low.

In most of the world a speedometer may read high (show more than true speed) by a set margin but must never read low. UN Regulation 39, used across Europe, allows an indicated speed up to 110% of true speed plus 4 km/h. A tire change that makes the dial read low can put the car outside that rule.

Yes — the odometer counts the same wheel revolutions. A taller tire means every logged mile is really a bit longer, so the odometer runs slow and the car accumulates fewer recorded miles than it actually travels. A shorter tire does the opposite.

On most cars since the mid-2000s a workshop can recalibrate the speedometer factor with a diagnostic tool. Older cars may need a physical correction box or a re-geared speedometer drive. Some aftermarket devices plug into the OBD port and apply the correction.