305/40R22 tyre size
A 305/40R22 tyre is 305 mm (12.0") wide with a sidewall height of 40 % of the width, i.e. 122.0 mm (4.8"), on a 22" (559 mm) rim. Overall diameter is 802.8 mm (31.6"), so the closest inch-style size is 31.6x12R22. Static circumference is 2522 mm; the ETRTO rolling circumference is 2449 mm, giving 408 revolutions per km (657 per mile).
Dimensions and rolling diameter
| Section width | 305 mm | 12.01″ |
| Sidewall height | 122.0 mm | 4.80″ |
| Rim diameter | 558.8 mm | 22″ |
| Overall diameter (static, unloaded) | 802.8 mm | 31.61″ |
| Static radius | 401.4 mm | 15.80″ |
| Static circumference (π × D) | 2,522 mm | 99.3″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,449 mm | 96.4″ |
| Effective rolling radius | 389.7 mm | 15.34″ |
| Revs per km (rolling / static) | 408 / 396 | |
| Inch equivalent | 31.6x12R22 | |
| Approved rim widths (approx.) | 10.5″–12″ (measuring rim 11″) | |
A 305/40R22 tyre is 305 mm (12.0") wide with a sidewall height of 40 % of the width, i.e. 122.0 mm (4.8"), on a 22" (559 mm) rim. Overall diameter is 802.8 mm (31.6"), so the closest inch-style size is 31.6x12R22. Static circumference is 2522 mm; the ETRTO rolling circumference is 2449 mm, giving 408 revolutions per km (657 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.