Tire Size Calculator
Compare two tire sizes using width, aspect ratio and rim diameter. See sidewall height, diameter, circumference, revolutions per mile and the theoretical effect on speedometer readings.
Your result
How the calculation works
Sidewall (mm) = width × aspect ratio/100. Diameter = rim inches × 25.4 + 2 × sidewall. Circumference = π × diameter. Revolutions per mile = 1,609,344/circumference. Actual speed = indicated speed × new diameter/original diameter.
Worked example
205/55R16 has a theoretical diameter of 631.9 mm. 225/45R17 is 634.3 mm, about 0.38% larger. A 60 mph indication would theoretically correspond to about 60.23 mph if calibration were unchanged.
Assumptions and limitations
This geometry does not establish safe fitment. Tire maker dimensions, wear, pressure, load, rim width and rolling radius differ. Offset, suspension clearance, traction controls, load rating, speed rating and legal requirements are outside the model.
Confirm compatibility with the vehicle/tire manufacturer and a qualified tire professional. No percentage difference shown here is an approval threshold. The diagram compares geometric diameter, not a mounted tire’s actual shape.
Method reviewed 2026-10-08. Calculation standards · Report an issue