🛞Tire Size Calculator

About this tool

Type a metric tyre size such as 225/45R17. Prefixes like P and speed letters like ZR are accepted. Add a second size to compare them. The calculator shows the section width, sidewall height, overall diameter, circumference and revolutions per kilometre and per mile for each, the diameter difference and what your true speed is when the speedometer reads 100.

How to read and compare tyre sizes

In 225/45R17, 225 is the section width in millimetres, 45 is the aspect ratio, meaning the sidewall is 45% of the width, R means radial construction, and 17 is the rim diameter in inches. The sidewall is 225 × 0.45 = 101.25 mm, so the overall diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm, about 25 inches. That wheel turns about 808 times per mile. Swapping to 225/40R18 keeps nearly the same diameter, 637.2 mm, just 0.46% larger, so at an indicated 100 you would really be doing about 100.5. Going to 245/45R17 makes the tyre 2.8% larger. Keep any change within about 3% of the original diameter. A larger tyre makes the speedometer read low, a smaller one makes it read high, and both affect the odometer, ABS and traction control. Honest limits: real dimensions vary slightly by brand and tread wear, and fitment also depends on load index, speed rating, wheel offset and clearance, so check the vehicle placard and ask a tyre fitter. The fuel cost calculator prices a trip, the unit converter switches between inches and millimetres and the percentage calculator handles other comparisons. Metric vs imperial: how we got two systems explains the mixed units.

Frequently asked questions