295/40R21 tyre size
A 295/40R21 tyre is 295 mm (11.6") wide with a sidewall height of 40 % of the width, i.e. 118.0 mm (4.6"), on a 21" (533 mm) rim. Overall diameter is 769.4 mm (30.3"), so the closest inch-style size is 30.3×11.6R21. Static circumference is 2417 mm; the ETRTO rolling circumference is 2347 mm, giving 426 revolutions per km (686 per mile).
Dimensions and rolling diameter
| Section width | 295 mm | 11.61″ |
| Sidewall height | 118.0 mm | 4.65″ |
| Rim diameter | 533.4 mm | 21″ |
| Overall diameter (static, unloaded) | 769.4 mm | 30.29″ |
| Static radius | 384.7 mm | 15.15″ |
| Static circumference (π × D) | 2,417 mm | 95.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,347 mm | 92.4″ |
| Effective rolling radius | 373.5 mm | 14.70″ |
| Revs per km (rolling / static) | 426 / 414 | |
| Inch equivalent | 30.3×11.6R21 | |
| Approved rim widths (approx.) | 10″–11.5″ (measuring rim 10.5″) | |
A 295/40R21 tyre is 295 mm (11.6") wide with a sidewall height of 40 % of the width, i.e. 118.0 mm (4.6"), on a 21" (533 mm) rim. Overall diameter is 769.4 mm (30.3"), so the closest inch-style size is 30.3×11.6R21. Static circumference is 2417 mm; the ETRTO rolling circumference is 2347 mm, giving 426 revolutions per km (686 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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