285/50R20 tyre size
A 285/50R20 tyre is 285 mm (11.2") wide with a sidewall height of 50 % of the width, i.e. 142.5 mm (5.6"), on a 20" (508 mm) rim. Overall diameter is 793.0 mm (31.2"), so the closest inch-style size is 31.2×11.2R20. Static circumference is 2491 mm; the ETRTO rolling circumference is 2419 mm, giving 413 revolutions per km (665 per mile).
Dimensions and rolling diameter
| Section width | 285 mm | 11.22″ |
| Sidewall height | 142.5 mm | 5.61″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 793.0 mm | 31.22″ |
| Static radius | 396.5 mm | 15.61″ |
| Static circumference (π × D) | 2,491 mm | 98.1″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,419 mm | 95.2″ |
| Effective rolling radius | 384.9 mm | 15.16″ |
| Revs per km (rolling / static) | 413 / 401 | |
| Inch equivalent | 31.2×11.2R20 | |
| Approved rim widths (approx.) | 8″–10″ (measuring rim 9″) | |
A 285/50R20 tyre is 285 mm (11.2") wide with a sidewall height of 50 % of the width, i.e. 142.5 mm (5.6"), on a 20" (508 mm) rim. Overall diameter is 793.0 mm (31.2"), so the closest inch-style size is 31.2×11.2R20. Static circumference is 2491 mm; the ETRTO rolling circumference is 2419 mm, giving 413 revolutions per km (665 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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