The letters on tires normally indicate whether they are passenger or lightweight truck tires, if the tires are radial and their maximum speed capability. Tires that do not begin with a letter are Euro-metric tires, which have a different load-bearing capability than P-metric tires, according to Goodyear.
The first letter in the sequence found on tire sidewalls is normally a P, representing P-metric, or LT, which represents that tires meet U.S. requirements for passenger vehicles or were designed for lightweight trucks, explains Goodyear. The R that is often found after a series of numbers stands for "radial tire." The last letter represents the tire's maximum speed rating. An example is H, which means the tire has a maximum speed capability of 130 mph.