Work out speedometer error from tyre size instantly with clear inputs, formula shown and shareable results.
The speedometer counts wheel revolutions, so a larger tyre travels further per revolution and the true speed exceeds the indication. Tread wear works the other way, reducing diameter by twice the wear depth and making the reading slightly optimistic again.
Speedometer error
actual speed = indicated x new diameter / original diameter
Yes, but only slightly. Losing 4 mm of tread reduces diameter by 8 mm, about 1.3% on a 632 mm tyre.
Generally only sizes approved for the vehicle. A significantly different rolling radius affects braking systems, stability control and the accuracy of legally required instruments.