255/50R20 tyre size
A 255/50R20 tyre is 255 mm (10.0") wide with a sidewall height of 50 % of the width, i.e. 127.5 mm (5.0"), on a 20" (508 mm) rim. Overall diameter is 763.0 mm (30.0"), so the closest inch-style size is 30x10R20. Static circumference is 2397 mm; the ETRTO rolling circumference is 2327 mm, giving 430 revolutions per km (692 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 127.5 mm | 5.02″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 763.0 mm | 30.04″ |
| Static radius | 381.5 mm | 15.02″ |
| Static circumference (π × D) | 2,397 mm | 94.4″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,327 mm | 91.6″ |
| Effective rolling radius | 370.4 mm | 14.58″ |
| Revs per km (rolling / static) | 430 / 417 | |
| Inch equivalent | 30x10R20 | |
| Approved rim widths (approx.) | 7″–9″ (measuring rim 8″) | |
A 255/50R20 tyre is 255 mm (10.0") wide with a sidewall height of 50 % of the width, i.e. 127.5 mm (5.0"), on a 20" (508 mm) rim. Overall diameter is 763.0 mm (30.0"), so the closest inch-style size is 30x10R20. Static circumference is 2397 mm; the ETRTO rolling circumference is 2327 mm, giving 430 revolutions per km (692 per mile).
Why two circumferences? The static figure is simple geometry (π × diameter). Under load a radial tyre flattens and rolls about 3 % less per revolution; ETRTO defines the rolling circumference as 3.05 × the nominal diameter, which is what manufacturers quote and what speedo and gearing calculations should use.
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