Passenger Tires
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 the manufacturers to grade passenger car tires based on three performance factors: tread wear, traction, and temperature resistance. back to top
Tread Wear
The tread wear 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 tread wear 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. back to top
Traction
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. back to top
Temperature
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. back to top
Speed Rating
The speed rating indicator on the tire relates to the maximum service speed of the tire, and signify the safe top speed of a tire under perfect conditions. Common speed ratings are:
Q=100 MPH, 160km/h
S=112 MPH, 180km/h
T=118 MPH, 190km/h
U=124 MPH, 200km/h
H=130 MPH, 210km/h
V=149 MPH, 240km/h
Z=149 MPH, 240km/h and over
W=168 MPH, 270km/h
Y=186 MPH, 300km/h
Tire Balance
Out-of-balance wheels can rob tires of thousands of miles. For a tire to run true, the combined weight of the tire and wheel assembly must be distributed uniformly. A heavy spot in the tire and wheel assembly must be counterbalanced by precision-made wheel weights.
Improper balance could lead to cupping and excessive wear of the tread at the heavy spot. The tread will eventually cup out to the point where the tire will no longer be usable. In addition, the constant bouncing of the tire will lead to excessive tire wear. Have your car's wheel alignment professionally checked every six months.
Tire and wheel assemblies can be balanced in two ways -- statically and dynamically.
In static (up and down) balancing, the assembly is mounted on a freely moving spindle. If, after rotation, one point consistently pulls to the bottom, the assembly is showing a heavy spot; weights are placed diametrically opposite to counterbalance it. After the weights are added, the heavy spot is gone and the tire and wheel assembly will rotate on the spindle and stop in any position, without a heavy spot. On your car, a tire and wheel assembly that is out of balance will thump and cause tire flat spots where the heavy spots are.
Dynamic (two plane) balancing reveals whether a tire is unbalanced up and down and also whether one side of a tire and wheel assembly is heavier than the other. Dynamic unbalance is reflected in a noticeable vibration sometimes described as "shimmy." It is corrected by equalizing the weight on both sides of the tire and wheel assembly. Note: Radial tires should always be dynamically balanced.
Tires and wheels should be balanced when new tires are mounted on wheels for the first time, when a tire and wheel are put in another position on the car, when used tires are installed on existing wheels after flat repair, or any time the tire is dismounted. Balance should also be checked at the first sign of vibration, "shimmy", or unusual tread wear. Vibration or irregular tread wear may also be caused by misalignment or mechanical problems. A professional diagnosis will reveal the true cause.
Tire Rotation
Tires on automobiles and light trucks should be rotated at 5,000 to 7,000 miles, and again every 5,000 to 7,000 miles thereafter. Proper rotation is almost as important as proper air-pressure.
Following a regular rotation schedule during the life of the tire will mean more even tread wear and longer tire life, particularly for front-wheel-drive and four-wheel-drive cars and light trucks.
The wear rate on the front wheels of most front-wheel-drive cars is about two times faster than the rear tires. The majority of new cars sold are front-wheel-drive, and about 26 million, or 20 percent of the cars registered in the United States are front-wheel-drive. Whatever the vehicle, check tires periodically for early detection of irregular wear. If you see irregular wear, rotate the tires after determining the cause and correcting any misalignment, balance or contributing mechanical problem.
