Upon first sight of a tyre you may notice number and letters stamped on the sidewall of the tyre. These numbers relate to the size of the tyre, for example: 255/70R22.5.
The first three numbers (255) indicate the section width of the tyre in millimetres, which is the width of the tyre from sidewall to sidewall.
The second numbers, in this case 70, denote the aspect ratio of the tyre which is the height of the tyre sidewall measured as a percentage of the width.
The R stands for Radial Construction.
This is the diameter of the wheel (in inches) that the tyre has been designed to fit on, in this example the rim diameter is 22.5 inches.