Tire size and rolling radius: calculate a speedometer change
Decode a size such as 225/55 R17 and compare diameter, circumference and indicated speed.
What to calculate
In 225/55 R17, 225 is nominal width in millimetres, 55 is sidewall height as a percent of width and 17 is rim diameter in inches. Approximate sidewall height = 225 × 0.55 = 123.75 mm. Convert the rim: 17 × 25.4 = 431.8 mm. Approximate overall diameter = 431.8 + 2 × 123.75 = 679.3 mm. Radius is half that, or 339.65 mm. An unloaded geometric radius is not the same as the effective rolling radius under vehicle load.
Worked example
Compare 225/55 R17 with 235/50 R18. The new sidewall is 117.5 mm, rim is 457.2 mm, and approximate diameter is 692.2 mm. The diameter change is (692.2 ÷ 679.3 − 1) × 100 ≈ 1.90%. If the speedometer was calibrated to the first size and reads 100 km/h, simple circumference scaling suggests about 101.9 km/h actual on the second. Real speedometer bias and tyre deformation can change that relationship.
Common mistake
A matching diameter does not guarantee fitment. Check manufacturer-approved sizes, wheel width, load and speed ratings, clearance, pressure recommendations and local requirements. Tread wear, tyre construction and inflation change effective rolling circumference. The result is an estimate; never use it to justify ignoring a speed limit.
How to use the result
Use the tire comparison calculator to contrast original and proposed sizes, the diameter calculator for geometry and annual mileage for distance-related planning. Check the proposed tyre against the vehicle handbook and a qualified fitter before purchase.
Related tools and reading
Our tire size and speedometer guide covers why indicated and actual distance can diverge.