285/40R21 tyre size
A 285/40R21 tyre is 285 mm (11.2") wide with a sidewall height of 40 % of the width, i.e. 114.0 mm (4.5"), on a 21" (533 mm) rim. Overall diameter is 761.4 mm (30.0"), so the closest inch-style size is 30×11.2R21. Static circumference is 2392 mm; the ETRTO rolling circumference is 2322 mm, giving 431 revolutions per km (693 per mile).
Dimensions and rolling diameter
| Section width | 285 mm | 11.22″ |
| Sidewall height | 114.0 mm | 4.49″ |
| Rim diameter | 533.4 mm | 21″ |
| Overall diameter (static, unloaded) | 761.4 mm | 29.98″ |
| Static radius | 380.7 mm | 14.99″ |
| Static circumference (π × D) | 2,392 mm | 94.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,322 mm | 91.4″ |
| Effective rolling radius | 369.6 mm | 14.55″ |
| Revs per km (rolling / static) | 431 / 418 | |
| Inch equivalent | 30×11.2R21 | |
| Approved rim widths (approx.) | 9.5″–11″ (measuring rim 10″) | |
A 285/40R21 tyre is 285 mm (11.2") wide with a sidewall height of 40 % of the width, i.e. 114.0 mm (4.5"), on a 21" (533 mm) rim. Overall diameter is 761.4 mm (30.0"), so the closest inch-style size is 30×11.2R21. Static circumference is 2392 mm; the ETRTO rolling circumference is 2322 mm, giving 431 revolutions per km (693 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.
Buying tyres for your business?
Onyx Tyres supplies workshops, retailers and fleets across Australia at wholesale prices. Open a trade account to see stock and pricing and order online.