Tyre sizes for a
18-inch rim

How the figures are derived

1. Read the size as the specification it is: section width in millimetres, aspect ratio as a percentage of that width, rim diameter in inches.
2. Sidewall height = width × aspect ÷ 100. This is the number the size string is really about, and it is the one nobody quotes.
3. Overall diameter = rim × 25.4 + two sidewalls — one at the top of the wheel and one at the bottom.
4. Circumference = π × overall diameter, which gives revolutions per kilometre and per mile directly.
5. 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 →