Size 305/40 R21
On the sidewall of the tyre there is a series of characters which are the tyre size, e.g. 305/40 R21, where the first number is the section width measured between the tyre sidewalls in millimetres. The second number in the tyre size is the profile height, which is a percentage of the tyre width, e.g. 55 is 55% of 205, or 112.75 mm. "R" indicates the type of tyre (R - radial, D - diagonal). The last number indicates the width of the rim in inches. Knowing your tyre size is essential when purchasing.