315 mm tyre sizes
12.40″ A 315 tyre is 315 millimetres — 12.40″ — across its widest point, mounted and inflated. Section width, not tread width: the bulge of the sidewall counts.
It appears in 75 sizes here, and what changes between them is the sidewall: same width, different aspect ratio, different rim, different overall height.
Every 315 mm size, shortest to tallest
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·all 75 designations at 315 mm section width·REVIEWED ETRTO STANDARD
How the figures are derived
- Read the size as the specification it is: section width in millimetres, aspect ratio as a percentage of that width, rim diameter in inches.
- Sidewall height = width × aspect ÷ 100. This is the number the size string is really about, and it is the one nobody quotes.
- Overall diameter = rim × 25.4 + two sidewalls — one at the top of the wheel and one at the bottom.
- Circumference = π × overall diameter, which gives revolutions per kilometre and per mile directly.
- Nothing above is measured, estimated, averaged or dated. It is the definition of the designation, so it does not go stale and it has no source to cite beyond the standard itself.
sidewall (mm) = width × aspect ÷ 100 overall diameter (mm) = rim × 25.4 + 2 × sidewall speedo true speed = indicated × (new diameter ÷ old diameter)
The full method, the ±3% rule, and what geometry cannot tell you →
About 315 mm tyres
How wide is a 315 tyre in inches?
12.40″. The 315 is the section width in millimetres, measured at the widest point of the mounted, inflated tyre — which includes the sidewall bulge and is not the same as the tread width.
What sizes come in 315 width?
75 of them across rims from 13 to 22 inches, ranging from 539 mm to 855 mm in overall diameter.