Free fitment tool
TYRE SIZE CALCULATOR
Compare two tyre sizes side by side - diameter, sidewall, circumference, speedometer error and the 3% rule.
Size 1 - original / OEM
/R
Size 2 - new tyre
/R
The size is printed on the tyre sidewall, for example 285/60R20.
Visual comparison
Comparison
| Size 1 | Size 2 | Difference | |
|---|---|---|---|
| Overall diameter | 850 mm (33.5") | 811 mm (31.9") | -4.6% |
| Width | 285 mm (11.2") | 275 mm (10.8") | -3.5% |
| Sidewall | 171 mm (6.7") | 151 mm (6.0") | -11.5% |
| Circumference | 2.67 m | 2.55 m | -4.6% |
| Revs per km | 374 | 393 | +19 |
Outside the 3% rule (-4.6%) - your speedometer will read high.
Speedometer error
| Speedo reads | 60 | 80 | 100 | 120 | 140 |
|---|---|---|---|---|---|
| True speed | 57.2 | 76.3 | 95.4 | 114.4 | 133.5 |
All speeds in km/h. Size 1 is treated as OEM.
