Tyre size calculator
Diameter, sidewall and revs per mile, and compare two sizes for speedometer difference.
225/45 R17 vs 205/55 R16 diameter
+0.38%
a very close match
- Sidewall
- 112.8 → 101.3 mm
- Diameter
- 631.9 → 634.3 mm
- Circumference
- 1,985.2 → 1,992.7 mm
- Revs per mile
- 811 → 808
| Speedometer shows | Real speed |
|---|---|
| 30 mph | 30.1 mph |
| 50 mph | 50.2 mph |
| 70 mph | 70.3 mph |
Show workings
- Sidewall205 × 55% = 112.8 mm
- Diameter16 in × 25.4 + 2 × 112.8 = 631.9 mm
- Difference(634.3 − 631.9) ÷ 631.9 = 0.38%
How it works
A tyre size like 205/55 R16 means a tyre 205 mm wide, with sidewalls 55% as tall as the width, on a 16-inch wheel.
- Sidewall = 205 × 0.55 = 112.75 mm
- Overall diameter = 16 × 25.4 + 2 × 112.75 = 631.9 mm
- Circumference = π × diameter; revolutions per mile = 1,609,344 mm ÷ circumference
A bigger overall diameter covers more ground per turn, so the speedometer (set up for the original size) reads lower than your real speed.
Worked example
Swapping 205/55 R16 (631.9 mm) for 225/45 R17 (634.3 mm) makes the wheel 0.4% bigger. At an indicated 70 mph you'd really be doing about 70.3 mph: a close match.
Questions
How much difference is acceptable?
A common guide is to stay within about 3% of the original overall diameter, and ideally within 1–2%. Always check the load index, speed rating and that the tyre fits the wheel arch, and tell your insurer about modifications.
Is a speedometer allowed to read high?
Speedometers may show more than your real speed but must never show less. That's why fitting bigger wheels and tyres needs care: they can make the speedometer under-read.
What do the other markings mean?
In "205/55 R16 91V", R means radial construction, 91 is the load index (615 kg per tyre) and V is the speed rating (up to 149 mph). Keep these at least as high as the vehicle requires.