Shown here is the sidewall of a popular "P-metric," speed-rated auto tire. "P" stands for passenger, "215" represents the width of the tire in millimeters; "65" is the ratio of height to width; "H" is the speed rating; "R" means radial; and "15" is the diameter of the wheel in inches. Some speed-rated tires carry a Service Description, instead of showing the speed symbol in the size designation. The Service Description, 89H in this example, consists of the load index (89) and speed symbol (H).

A "B" in place of the "R" means the tire is belted bias construction. A "D" in place of the "R" means diagonal bias construction.

The maximum load is shown in lbs. (pounds) and in kg (kilograms), and maximum pressure in psi (pounds per square inch) and in kPa (kilopascals). Kilograms and kilopascals are metric units of measurement.

The letters "DOT" certify compliance with all applicable safety standards established by the Department of Transportation (DOT).

Adjacent to this is a tire identification or serial number. This serial number is a code with up to 11 digits that are a combination of numbers and letters.

The sidewall also shows the type of cord and number of plies in the sidewall and under the tread. The DOT requires tire manufacturers to grade passenger car tires based on three performance factors: treadwear, traction and temperature resistance.

The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test track.

A tire graded 200 would wear twice as long on the government test course under specified test conditions as one graded 100.

It is wrong to link treadwear grades with your projected tire mileage. The relative performance of tires depends upon the actual conditions of their use and may vary due to driving habits, service practices, differences in road characteristics and climate.

Traction grades, from highest to lowest, are A, B and C. They represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete.

The temperature grades, from highest to lowest, are A, B and C. These represent the tire's resistance to the generation of heat when tested under controlled conditions on a specified indoor laboratory test wheel.

Here is the typical information on the sidewall of a light truck tire:

• "LT" stands for Light Truck. "LT235/85R16" is the size designation for a metric light truck tire.

• "LOAD RANGE D" identifies the load and inflation limits.

• "RADIAL" indicates that the tire has a radial construction.

• "MAX LOAD SINGLE 2623 lbs. AT 65 psi COLD" indicates the maximum load rating of the tire and corresponding minimum cold inflation pressure for that load when used as a single. For normal operation, follow pressure recommendations in owner's manual or on vehicle placard; "MAX LOAD DUAL 2381 lbs. AT 65 psi COLD" indicates the maximum load rating of the tire and corresponding minimum cold inflation pressure when used in a dual configuration. The other markings on the sidewall have the same meaning as described for the passenger car tire.