Three values describe the nominal shape
In 225/45R18, 225 is the nominal section width in millimeters, 45 is the sidewall height as a percentage of that width, and 18 is the rim diameter in inches. The letter R identifies radial construction. TireSpan uses the three dimensions for its geometry; it does not translate them into load capacity.
A worked example
For 225/45R18, the sidewall height is 225 × 0.45 = 101.25 mm. The rim diameter is 18 × 25.4 = 457.2 mm. Add one sidewall above and one below the rim to obtain 659.7 mm overall diameter. Circumference is 659.7 × π = 2,072.51 mm when rounded for display.
Keep the distinction between a label and a measurement
These values come from the marking, rather than a measurement of a particular tire. A manufacturer’s specification may report different measured width, diameter or revolutions per mile. Keep the tire model, measuring rim and test conditions with any such figure.
Sources and further reading
- Tire Markings Explained — Michelin
- Tire glossary — Michelin