Free Tire Circumference calculator with clear step-by-step results.
Derives tyre diameter from the sidewall marking - aspect ratio is a percentage of section width, not an absolute measurement - and then reduces the geometric circumference by a loaded deflection factor, because a tyre under load has a slightly shorter effective rolling circumference than its free diameter implies.
Overall diameter
Diameter = rim inches x 25.4 + 2 x (section width x aspect ratio)
Rolling circumference
Rolling = pi x diameter x (1 - loaded deflection)
The contact patch flattens, so the wheel centre travels slightly less per revolution than the free circumference. Two to four percent is typical for a passenger tyre at correct pressure.
Yes. Changing to a tyre with a different rolling circumference alters indicated speed and odometer readings in direct proportion.