There is a set of characters 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 the profile (height of the sidewall); the profile is a percentage value of the tyre’s width, e.g. 55 is 55% from 205, which is 112,75 mm. “R” indicates the construction type of a tyre (R – radial, - – diagonal). The last digit shows the size of the wheel you are to put a tyre on. It is essential to know the tyre size.