In order to establish what tyre size you need, just take a look at the tyre’s sidewall. The size may be written in two formats, for example, “18.4R26” or “420/80R46”. The first example here (18.4R26) is the imperial size expressed in inches, 18.4 being the section width of the tyre and 26 the rim size. The second example is the same size but expressed using metric measurement, 420 being the section width of the tyre, 80 representing the profile percentage (the side wall being 80% of the section width) and 26 being the rim size.
In the above example, the tyre we refer to is of radial construction. This is denoted by the “R” within the size description. In the following example, “18.4-26”, the dash symbol signifies that the tyre is of crossply (bias) construction. Please note that radial and crossply tyres offer different construction and performance characteristics.
If we take a popular turf size such as 20X10.00-8, the first part of this size description refers to the overall diameter (20 inches). The second part refers to the section width of the tyre (10 inches) and the final part refers to the rim size (8 inches).