255/45R20 tyre size
A 255/45R20 tyre is 255 mm (10.0") wide with a sidewall height of 45 % of the width, i.e. 114.8 mm (4.5"), on a 20" (508 mm) rim. Overall diameter is 737.5 mm (29.0"), so the closest inch-style size is 29x10R20. Static circumference is 2317 mm; the ETRTO rolling circumference is 2249 mm, giving 445 revolutions per km (715 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 114.8 mm | 4.52″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 737.5 mm | 29.04″ |
| Static radius | 368.8 mm | 14.52″ |
| Static circumference (π × D) | 2,317 mm | 91.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,249 mm | 88.6″ |
| Effective rolling radius | 358.0 mm | 14.09″ |
| Revs per km (rolling / static) | 445 / 432 | |
| Inch equivalent | 29x10R20 | |
| Approved rim widths (approx.) | 8″–9.5″ (measuring rim 8.5″) | |
A 255/45R20 tyre is 255 mm (10.0") wide with a sidewall height of 45 % of the width, i.e. 114.8 mm (4.5"), on a 20" (508 mm) rim. Overall diameter is 737.5 mm (29.0"), so the closest inch-style size is 29x10R20. Static circumference is 2317 mm; the ETRTO rolling circumference is 2249 mm, giving 445 revolutions per km (715 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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