If your vehicle is new or you always let the tire dealer tell you what tires to buy, you may not think reading a tire is important. After all, a set can last as long as 80,000 miles or six years, so by the time your current treads need replacing, you may no longer even own the vehicle they are mounted on. However, you could be away on a trip, get a blowout and need to buy a replacement tire. If this happens, you need to be sure that the replacement tire is the right one, even though it may not be the same brand as the other tires on the vehicle.

Another reason being able to read a tire is important is if you've just purchased a used car or truck. Unless you are able to read a sidewall, how will you know if the previous owner installed the correct tires?

Reading the sidewall
The numbers and letters on the tire's sidewall list the brand name and model, tire size, maximum load rating, maximum inflation level, whether the tire is a radial or bias design, and whether it is a tube or tubeless tire. They also tell tire ply composition and materials used, the U.S. Department of Transportation safety code, tread wear indicator, traction and temperature grade, date of manufacture and give a safety warning. These codes are standardized under the Uniform Tire Quality Grading (UTQG) system. All brands and types of tires are marked in the same way.

Look at your tires and you'll notice that the manufacturer's name and tire model (such as "Goodyear Eagle GT Plus 4"), along with a number designation that denotes tire size appear in large letters. A typical sidewall, for example, might have P205/60R15 89H molded into the surface. In this example, the "P" means the tire is designed for use on passenger cars (though it might also be used on some light trucks). The "205" is the width of the tire in millimeters, the "60" refers to the ratio of the tire's height to width, and "15" is the diameter of the tire in inches. The "R" means that the tire is a radial. The "89" is the load rating. A load rating of "89" means the tire is rated to carry a maximum of 1279 pounds. The "H" is the vehicle's speed rating (up to 130 mph).

If the size marking on your tire begins with "LT" instead of a "P", it means that the tire is rated for use on light trucks (pickups, vans, sport utilities) and a "B" or "D" in place of the "R" tells you the tire has belted bias or diagonal bias construction. Although all passenger and light-truck tires installed as original equipment are radials, many trailer and specialized-use tires are of bias or diagonal bias design.

Treadwear
The treadwear code is a three-digit number, such as "TREADWEAR 100." It is a comparative rating that indicates expected tread life. The base rating on passenger tires is 100, which translates to an expected tread life of 30,000 miles. Treadwear numbers go up in increments of 10, with each increase indicating an increase in tread life over the base rating. A tire with a treadwear rating of 150, for example, means it should have a 50 percent longer tread life, or 45,000 miles and a treadwear rating of 220, means the expected tread life is about 80,000 miles. Just because a tire has a specific treadwear rating, you may not actually get the rated mileage out of the tire. Actual tire wear will vary, depending upon vehicle type, driving conditions, maintenance, climate, road type, and driving habits.

On the newer tires, the letters "TWI" are imprinted at various locations around the sidewall, below the edge of the tread. This indicates the location of the Tread Wear Indicator bars that run across the tread pattern to indicate when the tread has worn down to unsafe levels. When these bars show, buy new tires.

Traction rating
A tire's gripping ability is rated as "AA," "A," "B," or "C," with "AA" being the highest. These letters represent the tire's ability to stop on wet pavement as measured under controlled conditions on a test track. A tire rated as "AA" has superior wet braking traction (usually reserved for specially-designed rain tires). An "A" grade signifies that the tire has excellent wet braking traction, "B" is the middle performance standard, and "C" is the lowest traction grade. Because a tire has a "B" or a "C" grade doesn't mean it has poor traction. What it really means is that you should pay attention to the type of weather and road conditions in your area. If you live in Arizona, for example, a C-rated tire may provide excellent traction for weather and road conditions that are usually dry. In the rainy Pacific Northwest, however, a tire with an "A" or "AA" rating may be a better choice.

Speed rating
The tires on your vehicle will be marked with an "S", "T", "H", "V", or "Z." These symbols indicate a tire's speed capability based upon laboratory tests under ideal conditions. While the speed rating is a good indicator of the speeds at which the tire is safe to operate, the ratings become invalid if the tire is underinflated, worn, damaged or overloaded.

Most economy cars and sedans have tires rated at "S" or "T" (up to 112 mph). Performance sedans and sports cars have tires rated "H" (up to 130 mph), "V" (up to 149 mph), or "Z" (150 mph and over), depending upon the vehicle's capability. Just because a tire is rated at a certain speed, however, doesn't mean you can drive safely at that speed. A sports car fitted with "Z"-rated tires, for example, cannot be safely driven at the speed for which the tire is rated because our roads aren't in good enough condition. In addition, traffic volume, weather, vehicle condition, driver ability and a maximum speed limit of 70 mph, make driving this fast foolhardy.

Load rating
The amount of weight a tire can safely carry is its load rating. Passenger car tires have a number rating between 65 (639 lbs.) and 104 (1,984 lbs.), with most passenger tires rated between 75 and 100. Knowing how to read load ratings isn't that important because the maximum load rating, in pounds, is stamped in small letters on the sidewall near the edge of the wheel rim.

Light-truck tires are rated differently and may not have a load index or speed rating in the size designation. Instead, letters like "M+S" (mud and snow) or A/T (all-terrain) indicate the tire's intended use at the end of the size listing. This is because truck tires are designed for load-carrying ability, rather than high-speed performance. Load index is listed alphabetically, from "A" to "E," with "E" being highest. A compact pickup may have "A"-rated tires because it isn't designed as a vehicle that regularly carries heavy loads, while a full-size Suburban with a Class IV hitch is designed to haul heavy loads and will probably have "E"-rated tires.

DOT Standard Safety Code
Every tire has a Department of Transportation Standard Safety Code Number. A typical number might read "DOT MA L9 ABC 0301," with "DOT" standing for Department of Transportation. "MA" is the manufacturer's plant and code number. "L9" is the tire size code number (P215/65R15). "ABC" is a manufacturer optional code that identifies the tire and "0301" is the date of manufacture, in this case the 3rd week of 2001. (Tires manufactured before January 2000 use a three-digit date code.)