275/40R21 tyre size
A 275/40R21 tyre is 275 mm (10.8") wide with a sidewall height of 40 % of the width, i.e. 110.0 mm (4.3"), on a 21" (533 mm) rim. Overall diameter is 753.4 mm (29.7"), so the closest inch-style size is 29.7×10.8R21. Static circumference is 2367 mm; the ETRTO rolling circumference is 2298 mm, giving 435 revolutions per km (700 per mile).
Dimensions and rolling diameter
| Section width | 275 mm | 10.83″ |
| Sidewall height | 110.0 mm | 4.33″ |
| Rim diameter | 533.4 mm | 21″ |
| Overall diameter (static, unloaded) | 753.4 mm | 29.66″ |
| Static radius | 376.7 mm | 14.83″ |
| Static circumference (π × D) | 2,367 mm | 93.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,298 mm | 90.5″ |
| Effective rolling radius | 365.7 mm | 14.40″ |
| Revs per km (rolling / static) | 435 / 422 | |
| Inch equivalent | 29.7×10.8R21 | |
| Approved rim widths (approx.) | 9″–10.5″ (measuring rim 9.5″) | |
A 275/40R21 tyre is 275 mm (10.8") wide with a sidewall height of 40 % of the width, i.e. 110.0 mm (4.3"), on a 21" (533 mm) rim. Overall diameter is 753.4 mm (29.7"), so the closest inch-style size is 29.7×10.8R21. Static circumference is 2367 mm; the ETRTO rolling circumference is 2298 mm, giving 435 revolutions per km (700 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.