Tyre Size Calculator
Calculate tyre sidewall, overall diameter, circumference and revs-per-mile from standard tyre size codes (e.g. 205/55 R16). Useful for size swaps and speedo errors.
Last updated: May 2026
Enter a tyre size to calculate diameter and circumference.
Reading tyre size markings
A standard tyre size like 225/45 R17 encodes three measurements. The first number is tyre width in millimetres (225 mm). The second is the aspect ratio, sidewall height as a percentage of width (45% of 225 mm = 101.25 mm sidewall). R indicates radial construction. The final number is rim diameter in inches (17 inches = 431.8 mm). Overall diameter = rim diameter + 2 × sidewall height.
For 225/45 R17: overall diameter = 431.8 + 2 × 101.25 = 634.3 mm. Circumference = π × 634.3 mm = 1,992 mm per rotation. At 60 km/h, the wheel rotates approximately 838 times per minute.
Common tire sizes and overall diameter
| Tire size | Width (mm) | Sidewall (mm) | Overall diam. (mm) |
|---|---|---|---|
| 195/65 R15 | 195 | 127 | 635 |
| 205/55 R16 | 205 | 113 | 632 |
| 215/60 R16 | 215 | 129 | 664 |
| 225/45 R17 | 225 | 101 | 634 |
| 235/35 R19 | 235 | 82 | 647 |
| 255/40 R18 | 255 | 102 | 661 |
Worked example: tyre size swap and speedometer error
Suppose your car came with 205/55 R16 tyres and you want to fit 225/45 R17 wheels. The tyre width and rim diameter both increase, so it is worth checking whether the overall diameter stays close enough to keep your speedometer honest. Work through both sizes step by step.
Original size, 205/55 R16. Sidewall height = 205 mm × 0.55 = 112.75 mm. Rim diameter = 16 inch × 25.4 = 406.4 mm. Overall diameter = 406.4 + 2 × 112.75 = 631.9 mm.
New size, 225/45 R17. Sidewall height = 225 mm × 0.45 = 101.25 mm. Rim diameter = 17 inch × 25.4 = 431.8 mm. Overall diameter = 431.8 + 2 × 101.25 = 634.3 mm.
Percentage difference. (634.3 - 631.9) / 631.9 = 2.4 / 631.9 = 0.0038, which is 0.38%. That is well inside the 2 to 3% tolerance most carmakers accept, so the swap is dimensionally safe.
Speedometer error. Because the speedometer was calibrated for the smaller 631.9 mm tyre, the larger 634.3 mm tyre travels 0.38% further per rotation than the gauge assumes. When the speedometer reads 100 km/h, your true speed is 100 × (634.3 / 631.9) = 100.38 km/h. When it reads 50 mph, you are actually doing about 50.19 mph. The error is tiny here, but the same arithmetic shows why a much taller tyre, for example jumping two rim sizes, can make the speedometer read several percent slow.
Frequently Asked Questions
How do I read a standard tyre size like 225/45 R17?
225 = tyre width in mm. 45 = aspect ratio (sidewall height is 45% of 225 mm = 101.25 mm). R = radial construction. 17 = rim diameter in inches. Overall diameter = rim (17" × 25.4 = 431.8 mm) + 2 × sidewall (101.25 mm) = 634.3 mm total.
Does changing tyre size affect my speedometer?
Yes. The speedometer reads speed from wheel rotation count. A larger overall diameter means the tyre travels further per rotation, so actual speed is higher than displayed. Keep overall diameter within 2 to 3% of the original size for accurate speed readings. ABS, traction control and odometer accuracy are also affected.
What is tyre circumference used for?
Distance per rotation. Useful for calculating the effect of tyre size changes on odometer readings, ABS pulse intervals, and GPS odometry calibration. It also affects rev counting in performance driving, a larger circumference means fewer rotations per kilometre, which feels different under acceleration.