Cooper Discoverer S/T Maxx LT35x12.50 R20 121Q 10PR, POR, studdable

The exact dimensions of a tyre can be found on the sidewall printed mark. The graphic shows how to read properties and dimensions of the tyre:

PR is short for "ply-rating". This label originally described the number of cotton cord layers in the casing. A higher number of cord layers means a better carrying capacity. When stronger materials were introduced for the cord layers, this term was removed from the actual number of cord layers. Now, a tyre with 8 PR could have only four nylon cord layers which equalled the strength of eight cotton cord layers. Today, the stability of street-worthy tyres is expressed through the load and speed index instead of the ply rating.