Take the 2-minute tour ×  Is it true that 26" rim actual
diameter is 25.625"? How does tire size affect wheel diameter?
Is there any special formula to calculate wheel diameter?
share|improve this question  3 Answers 3  up vote 5 down vote
accepted  You could calculate the effective diameter by
measuring rollout and dividing that number by pi. To measure the
rollout start with your valve stem in the lowest possible
position and make a mark on the floor next to the valve stem.
Ride the bike one revolution until the valve stem is at the
bottom of it's rotation again and make another mark on the
floor. Measure the distance between the two marks to find the
effective circumference.  share|improve this answer Way to
provide the physical/math answer and satisfy my math jones (see
bicycles.stackexchange.com/questions/1549/…) –  Gary.Ray May 3
'11 at 19:50 This is actually the only accurate way to know.
There are too many variables to allow you to read numbers off
the tyre and get an accurate answer. @Mike Baranczak