245/40R20 tyre size
A 245/40R20 tyre is 245 mm (9.6") wide with a sidewall height of 40 % of the width, i.e. 98.0 mm (3.9"), on a 20" (508 mm) rim. Overall diameter is 704.0 mm (27.7"), so the closest inch-style size is 27.7×9.6R20. Static circumference is 2212 mm; the ETRTO rolling circumference is 2147 mm, giving 466 revolutions per km (750 per mile).
Dimensions and rolling diameter
| Section width | 245 mm | 9.65″ |
| Sidewall height | 98.0 mm | 3.86″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 704.0 mm | 27.72″ |
| Static radius | 352.0 mm | 13.86″ |
| Static circumference (π × D) | 2,212 mm | 87.1″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,147 mm | 84.5″ |
| Effective rolling radius | 341.7 mm | 13.45″ |
| Revs per km (rolling / static) | 466 / 452 | |
| Inch equivalent | 27.7×9.6R20 | |
| Approved rim widths (approx.) | 8″–9.5″ (measuring rim 8.5″) | |
A 245/40R20 tyre is 245 mm (9.6") wide with a sidewall height of 40 % of the width, i.e. 98.0 mm (3.9"), on a 20" (508 mm) rim. Overall diameter is 704.0 mm (27.7"), so the closest inch-style size is 27.7×9.6R20. Static circumference is 2212 mm; the ETRTO rolling circumference is 2147 mm, giving 466 revolutions per km (750 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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