275/45R21 tyre size
A 275/45R21 tyre is 275 mm (10.8") wide with a sidewall height of 45 % of the width, i.e. 123.8 mm (4.9"), on a 21" (533 mm) rim. Overall diameter is 780.9 mm (30.7"), so the closest inch-style size is 30.7×10.8R21. Static circumference is 2453 mm; the ETRTO rolling circumference is 2382 mm, giving 420 revolutions per km (676 per mile).
Dimensions and rolling diameter
| Section width | 275 mm | 10.83″ |
| Sidewall height | 123.8 mm | 4.87″ |
| Rim diameter | 533.4 mm | 21″ |
| Overall diameter (static, unloaded) | 780.9 mm | 30.74″ |
| Static radius | 390.5 mm | 15.37″ |
| Static circumference (π × D) | 2,453 mm | 96.6″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,382 mm | 93.8″ |
| Effective rolling radius | 379.1 mm | 14.92″ |
| Revs per km (rolling / static) | 420 / 408 | |
| Inch equivalent | 30.7×10.8R21 | |
| Approved rim widths (approx.) | 8.5″–10″ (measuring rim 9″) | |
A 275/45R21 tyre is 275 mm (10.8") wide with a sidewall height of 45 % of the width, i.e. 123.8 mm (4.9"), on a 21" (533 mm) rim. Overall diameter is 780.9 mm (30.7"), so the closest inch-style size is 30.7×10.8R21. Static circumference is 2453 mm; the ETRTO rolling circumference is 2382 mm, giving 420 revolutions per km (676 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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