Definition of Tire size numbers
P =Passenger Car Tire
The first letter 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. A European metric tire would have no letter before the tire size.
245 =Section Width
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. It is a percentage of the section width. In this example, the sidewall height is 45% of the 245 section width which equals 127.5.
Z =Speed Rating
S = 112mph
T = 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, 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.
Uniform Tire Quality Grading (UTQG) Marking
The Department of Transportation requires each manufacturer to grade its tires under the Uniform Tire Quality Grade (UTQG) system which establishes ratings for treadwear, traction and temperature resistance. It is important to note that these tests are conducted independently by each manufacturer and not by the Department of Transportation.
The treadwear rating (440) refers to the durability of the tire, not necessarily the projected life of the tire. Since the test is conducted by each manufacturer, and the grade is assigned after the tire has only experienced a small amount of treadwear, there is room for interpretation. While it may be helpful to compare treadwear grades within the same manufacturer, it might not be as helpful to compare the grades across multiple brands or manufacturers.
The traction rating (A) refers to the tire’s ability to stop in a straight line on wet asphalt and concrete under the controlled conditions of the test track. The test does not measure the tire’s cornering ability or hydroplaning resistance in wet conditions or braking and handling in dry conditions. The traction grades consist of AA, A, B or C, with AA being the highest
The temperature rating (A) refers to the tire's resistance to the generation of heat when driven at high speeds. The grades range from A through C with A being the highest, making tires with an A grade most effective at dissipating heat.
Maximum Inflation Pressure and Load
This maximum inflation pressure number, (51 PSI) indicates the maximum air pressure the tire can hold while in operation. This does not represent the recommended inflation pressure for the vehicle it is installed on. You should always set your air pressure to your vehicle manufacturer’s specifications as stated in your owner’s manual or on the tire information placard inside the vehicle.
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