Service Description
Many tires come with a service description added on to the end of the tire's size. These service descriptions contain a two-digit number (load index) and a letter (speed rating).

The load index refers to the load-carrying capacity of a tire, or how much weight a tire can support. For example, if a tire has a load index of 89, it can support 1,279 pounds (from below chart) at maximum air pressure.  It is not recommended to install tires with a lower load index than what came on your car from the factory.