245/45R20 tyre size
A 245/45R20 tyre is 245 mm (9.6") wide with a sidewall height of 45 % of the width, i.e. 110.3 mm (4.3"), on a 20" (508 mm) rim. Overall diameter is 728.5 mm (28.7"), so the closest inch-style size is 28.7×9.6R20. Static circumference is 2289 mm; the ETRTO rolling circumference is 2222 mm, giving 450 revolutions per km (724 per mile).
Dimensions and rolling diameter
| Section width | 245 mm | 9.65″ |
| Sidewall height | 110.3 mm | 4.34″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 728.5 mm | 28.68″ |
| Static radius | 364.3 mm | 14.34″ |
| Static circumference (π × D) | 2,289 mm | 90.1″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,222 mm | 87.5″ |
| Effective rolling radius | 353.6 mm | 13.92″ |
| Revs per km (rolling / static) | 450 / 437 | |
| Inch equivalent | 28.7×9.6R20 | |
| Approved rim widths (approx.) | 7.5″–9″ (measuring rim 8″) | |
A 245/45R20 tyre is 245 mm (9.6") wide with a sidewall height of 45 % of the width, i.e. 110.3 mm (4.3"), on a 20" (508 mm) rim. Overall diameter is 728.5 mm (28.7"), so the closest inch-style size is 28.7×9.6R20. Static circumference is 2289 mm; the ETRTO rolling circumference is 2222 mm, giving 450 revolutions per km (724 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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