There is a set of digits on a tyre's sidewall, which indicates it size, e.g. 225/40 R18, where first number indicates the width measured in millimetres between outer and inner sidewall of the tyre. The second number indicates profile (height of the sidewall); that is a percentage value of the tyre’s width, e.g. 55 is 55% from 205, that is 112,75 mm. “R” indicated type of a tyre (R – radial, D – diagonal). Last digit shows the size of the rim you are to put a tyre on. It is essential to know the tyre size.