There are three basic elements represented in the tire size. They are:
For example 255/55R-18, P205/65R-15, 37x13.50R17E, etc.
Service description is a combination of load index and speed rating. For example 87Z, 92H, 109Q XL, etc.
Sidewall Description (not printed on sidewall)
Some tire sizes will also be displayed with the manufacturer's abbreviation describing the sidewall design. For example OWL, RBL, WW, etc.
For more detailed information on these and all the other markings on your tire, check out our Reading the Tire Size article in the Info Center.