Tire Codes and Ratings
Facts and figures to help keep you on track
Most of us rarely, if ever, think of our tires as long as they hold air. The time they enter our consciousness is usually when they're making the "flubbidy-dubbity" sound, signaling the end of their lifecycle—and a smooth trip. Yet, in the time it takes you—out there on the highway, in the rain, hazard lights flashing, changing your flat tire or watching the AAA person do it—you could be educating yourself.
Did you know that those letters and numbers on the sidewall of your tires offer all kinds of useful information?
Tire Codes
Molded into every tire sidewall is a series of codes that give valuable information to the consumer regarding that specific tire, such as name of the tire, its size, whether it is tubeless or its tube type, the tire grade, speed rating, the maximum load, maximum inflation, important safety warnings, etc. Let's look at a typical passenger car tire to see what those letters and number really mean:
Example: P255/60R15 102T
"P" means this is a passenger car tire (as opposed to a tire made for a truck or other vehicle). P-metric is the U.S. version of a metric tire-sizing system. LT designates the tire as a light truck (or SUV) tire.
"255" Section Width: The width of the tire in millimeters from sidewall to sidewall. This measurement varies depending on the width of the rim to which the tire is fitted: larger on a wider rim, smaller on a narrow rim. The number on the side of tire indicates the width measured with the tire fitted to the tire manufacturers recommended rim width.
"60" Aspect Ratio: The ratio of height to width (this tire's height is 60 percent of its width).
"R" Construction: How the plies are constructed in the tire carcass. "R" means radial. "B" in place of the "R" means the tire is belted bias construction. "D" in place of the "R" means diagonal bias construction. The vast majority of modern cars, SUVs, and trucks use radial tires.
"15" Rim Diameter: The diameter of the steel or aluminum wheel in inches the tire will fit.
"102" Load Index: This tire has an industry-standard maximum load of 1,874 lbs. Different numbers correspond to different maximum loads. 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).
"T" Speed Rating: This tire has an industry-standard maximum service speed of 118 mph. Tires using an older European system carry the speed rating in the size description: 255/60HR15. Different letters correspond to different maximum service speeds.
Speed Rating
The speed rating is the maximum service speed of a passenger car tire. Exceeding the maximum recommended speed can drastically reduce tire life, and may result in tire failure. Here is a chart of rating indicators and their mile-per-hour equivalents. The speed rating shown indicates the top speed for which a tire is certified. It does not indicate the total performance capability of the tire. This rating system applies to all tire makers, but does not apply to light truck tires, as speed ratings are not yet required by the DOT.
Rating Max Speed
Q 99 mph
S 112 mph
T 118 mph
U 124 mph
H 130 mph
V 149 mph
W 168 mph
Y 186 mph
Z Above 149 mph
The letters "DOT" certify compliance with all applicable safety standards established by the U.S. Department of Transportation (DOT). Next to this is a tire identification or serial number—basically a combination of numbers and letters with up to 11 digits. These are essentially production codes designating where and when the tire was produced.
Tire Grades
Uniform Tire Quality Grading System or UTQG. Except for snow tires, the DOT requires the manufacturers to grade passenger car tires based on three performance factors: tread wear, traction and temperature resistance.
Tread Wear
More Than 100—Better
100—Baseline
Less Than 100—Poorer
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 track as one graded 100. Your actual tire mileage depends upon the conditions of their use and variations in driving habits, service practices, differences in road characteristics and climate. It should be noted that tread wear grades are valid only for comparisons within a manufacturer's product line. They are not valid for comparisons between different manufacturers.
Traction
A—Best
B—Intermediate
C—Acceptable
Traction grades 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 Traction grade is based upon "straight ahead" braking tests; it does not indicate cornering ability.
Temperature
A—Best
B—Intermediate
C—Acceptable
The temperature grades represent the tire's resistance to the generation of heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperatures can cause the materials of the tire to degenerate and thus reduce tire life. Excessive temperatures can lead to tire failure. Federal law requires that all tires meet at least the minimal requirements of Grade C.
To ensure you get the most miles out of your tires, the Rubber Manufacturers Association recommends following their PARTS (Pressure, Alignment, Rotation, Tread) system:
Pressure: Under-inflation is a leading cause of tire failure. A tire can lose nearly half its recommended pressure before it appears flat. As pressure drops, heat increases, which can lead to a catastrophic failure (a.k.a. blowout).
Alignment: Bumping a curb or hitting a pothole can knock your front end out of alignment. Once out of alignment, the tire will wear much quicker and shorten its usable life. If, while in motion, your car "pulls" to one side as you release the steering wheel, chances are you need to have it re-aligned.
Rotation: Rotating your vehicle's tires will even the wear of the tires. Your vehicle owner's manual should outline the recommended mileage for rotations. The industry standard is 6,000 miles. Directional tires should only be rotated front to back, not side to side.
Tread: Unusual tread wear may be an indication of other problems, such as worn springs, worn shocks or struts, or an alignment problem. Check your tires periodically for uneven wear, tread depth or other signs of damage.
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