Speedometer accuracy with different wheel size?

[finagle69]

Originally Posted by Cavman

Thank you guys. Great info. Very informative.

Seems like a move from 19" to 20" is about a 0.7% increase in revs per mile (19" OEM tires are 716 revs/mile and 20" OEM tires are 721 revs/mile).

Therefore, if I am doing the math correctly, with the 20" tires and the speedometer reading 65mph, I will actually be traveling 65.455mph.

This doesn't take into consideration any "padding" that BMW has apparently added to our speedometer.

Thanks again.

That's a half truth.

Not all tires are the same OD for the same wheel size. This is why you can buy 315/25/20 or 315/30/20 or any other combination. The middle number is a percentage of the first number, so the higher the percentage, the larger the OD will be. Thus giving you an even lower rev/mile.

This is why the calculators can actually be helpful.