265/40R21 tyre size
A 265/40R21 tyre is 265 mm (10.4") wide with a sidewall height of 40 % of the width, i.e. 106.0 mm (4.2"), on a 21" (533 mm) rim. Overall diameter is 745.4 mm (29.3"), so the closest inch-style size is 29.3×10.4R21. Static circumference is 2342 mm; the ETRTO rolling circumference is 2273 mm, giving 440 revolutions per km (708 per mile).
Dimensions and rolling diameter
| Section width | 265 mm | 10.43″ |
| Sidewall height | 106.0 mm | 4.17″ |
| Rim diameter | 533.4 mm | 21″ |
| Overall diameter (static, unloaded) | 745.4 mm | 29.35″ |
| Static radius | 372.7 mm | 14.67″ |
| Static circumference (π × D) | 2,342 mm | 92.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,273 mm | 89.5″ |
| Effective rolling radius | 361.8 mm | 14.25″ |
| Revs per km (rolling / static) | 440 / 427 | |
| Inch equivalent | 29.3×10.4R21 | |
| Approved rim widths (approx.) | 9″–10.5″ (measuring rim 9.5″) | |
A 265/40R21 tyre is 265 mm (10.4") wide with a sidewall height of 40 % of the width, i.e. 106.0 mm (4.2"), on a 21" (533 mm) rim. Overall diameter is 745.4 mm (29.3"), so the closest inch-style size is 29.3×10.4R21. Static circumference is 2342 mm; the ETRTO rolling circumference is 2273 mm, giving 440 revolutions per km (708 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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