265/45R21 tyre size
A 265/45R21 tyre is 265 mm (10.4") wide with a sidewall height of 45 % of the width, i.e. 119.3 mm (4.7"), on a 21" (533 mm) rim. Overall diameter is 771.9 mm (30.4"), so the closest inch-style size is 30.4×10.4R21. Static circumference is 2425 mm; the ETRTO rolling circumference is 2354 mm, giving 425 revolutions per km (684 per mile).
Dimensions and rolling diameter
| Section width | 265 mm | 10.43″ |
| Sidewall height | 119.3 mm | 4.69″ |
| Rim diameter | 533.4 mm | 21″ |
| Overall diameter (static, unloaded) | 771.9 mm | 30.39″ |
| Static radius | 386.0 mm | 15.19″ |
| Static circumference (π × D) | 2,425 mm | 95.5″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,354 mm | 92.7″ |
| Effective rolling radius | 374.7 mm | 14.75″ |
| Revs per km (rolling / static) | 425 / 412 | |
| Inch equivalent | 30.4×10.4R21 | |
| Approved rim widths (approx.) | 8.5″–10″ (measuring rim 9″) | |
A 265/45R21 tyre is 265 mm (10.4") wide with a sidewall height of 45 % of the width, i.e. 119.3 mm (4.7"), on a 21" (533 mm) rim. Overall diameter is 771.9 mm (30.4"), so the closest inch-style size is 30.4×10.4R21. Static circumference is 2425 mm; the ETRTO rolling circumference is 2354 mm, giving 425 revolutions per km (684 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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