255/40R19 tyre size
A 255/40R19 tyre is 255 mm (10.0") wide with a sidewall height of 40 % of the width, i.e. 102.0 mm (4.0"), on a 19" (483 mm) rim. Overall diameter is 686.6 mm (27.0"), so the closest inch-style size is 27x10R19. Static circumference is 2157 mm; the ETRTO rolling circumference is 2094 mm, giving 478 revolutions per km (769 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 102.0 mm | 4.02″ |
| Rim diameter | 482.6 mm | 19″ |
| Overall diameter (static, unloaded) | 686.6 mm | 27.03″ |
| Static radius | 343.3 mm | 13.52″ |
| Static circumference (π × D) | 2,157 mm | 84.9″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,094 mm | 82.4″ |
| Effective rolling radius | 333.3 mm | 13.12″ |
| Revs per km (rolling / static) | 478 / 464 | |
| Inch equivalent | 27x10R19 | |
| Approved rim widths (approx.) | 8.5″–10″ (measuring rim 9″) | |
A 255/40R19 tyre is 255 mm (10.0") wide with a sidewall height of 40 % of the width, i.e. 102.0 mm (4.0"), on a 19" (483 mm) rim. Overall diameter is 686.6 mm (27.0"), so the closest inch-style size is 27x10R19. Static circumference is 2157 mm; the ETRTO rolling circumference is 2094 mm, giving 478 revolutions per km (769 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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