P =Passenger Car Tire
The first letter on a tire indicates the tire’s class. P designates this tire as a passenger car tire, an LT before the tire size would mean the tire is a light truck tire, and a European metric tire would have no letter before the tire size.
245 =Section WidthTread (mm)
The section width is the width of the tire from one sidewall to the other measured in millimeters. In this example, the tire is 245 millimeters wide.
45 =Aspect Ratio
(Ratio of Sidewall height to width)
The aspect ratio refers to the height of the sidewall, and it is a percentage of the section width. In this example, the sidewall height is 45% of the 245 section width which equals 110.25.
Z =Speed Rating S = 112mphT = 118mph
The speed rating designates the maximum speed at which a properly installed and inflated tire can be driven on. In this example, the speed rating is Z which means it has the highest speed rating. Z rated tires will also have a W or Y speed rating indication after the load index which denotes the actual mph the tire is rated for.
In this example, the R designates this tire as a radial tire. Radial tires have layers of fabric whose cords run at right angles to the circumference of the tire and whose tread is strengthened by additional layers around the circumference making it the most common type of automotive tire.
Wheel Diameter specifies the size, in inches, of the wheel that a tire fits. In this example, the tire will fit a 19 inch wheel.
Load Index indicates the maximum load carrying capacity of the tire. It is very important to only install tires with a load index that meets or exceeds your vehicle manufacturer's specification.
The speed rating designates the maximum speed at which a properly installed and inflated tire can be driven on. In this example, the speed rating is W which means it can be driven at speeds up to 168 mph.
U.S. Department of Transportation Markings
All tires must have a DOT, or Department of Transportation number, which indicates the tire has passed all minimum DOT standards for sale in the U.S. This code represents the manufacturer and the plant it was manufactured at (R8) and the size (LN). Also, an optional code for manufacturing (LMJR) which specifies the construction, tread pattern and category of the tire. The week (46) and year (10) the tire was produced are at the end of the code.