Speedometer Error Calculator

After a tire size change, find your real speed versus what the speedometer shows.

Formula

Actual speed = Speedometer reading (km/h) × [Diameter(New) / Diameter(Current)]

Assumptions

After a tire size change, find your real speed versus what the speedometer shows.

How to read it

A larger tire makes you drive faster than the speedometer shows; a smaller one, the opposite. The percentage applies at every speed and to the odometer too — useful to interpret real fuel economy and to stay within speed limits with a different fitment.

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