Free Tire Revolutions per Mile calculator with clear step-by-step results.
Revolutions per mile is the inverse of rolling circumference and is what drives speedometer calibration, ABS wheel-speed comparison and odometer accuracy. Comparing a new tyre's circumference against the original gives the scale error introduced by the change.
Revolutions per mile
Revs = 1,609,344 mm per mile / rolling circumference mm
Speedometer error
Error % = (new circumference / original circumference - 1) x 100
Changing tyre size affects speedometer accuracy, odometer readings, ABS and stability control calibration, and may breach vehicle construction and use regulations or insurance conditions.
A larger circumference means fewer revolutions for the same distance, so the speedometer under-reads and you are travelling faster than indicated.
Most regulations allow a speedometer to over-read but never under-read. Keeping a tyre change within about 1.5% of the original circumference avoids problems.