255/55R20 tyre size
A 255/55R20 tyre is 255 mm (10.0") wide with a sidewall height of 55 % of the width, i.e. 140.3 mm (5.5"), on a 20" (508 mm) rim. Overall diameter is 788.5 mm (31.0"), so the closest inch-style size is 31x10R20. Static circumference is 2477 mm; the ETRTO rolling circumference is 2405 mm, giving 416 revolutions per km (669 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 140.3 mm | 5.52″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 788.5 mm | 31.04″ |
| Static radius | 394.3 mm | 15.52″ |
| Static circumference (π × D) | 2,477 mm | 97.5″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,405 mm | 94.7″ |
| Effective rolling radius | 382.8 mm | 15.07″ |
| Revs per km (rolling / static) | 416 / 404 | |
| Inch equivalent | 31x10R20 | |
| Approved rim widths (approx.) | 7″–9″ (measuring rim 8″) | |
A 255/55R20 tyre is 255 mm (10.0") wide with a sidewall height of 55 % of the width, i.e. 140.3 mm (5.5"), on a 20" (508 mm) rim. Overall diameter is 788.5 mm (31.0"), so the closest inch-style size is 31x10R20. Static circumference is 2477 mm; the ETRTO rolling circumference is 2405 mm, giving 416 revolutions per km (669 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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