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A Little Good Reading

Your tire's sidewalls can tell you a lot, if you know how to read the code


by Peter D. du Pre

Although I frequent the bookstore on a regular basis to purchase automotive books, I also like books on non-automotive subjects, particularly mystery stories. I enjoy helping the hero read the clues and figure out "whodunnit." There's nothing like a good mystery story.

In fact, just the other day, I was curled up in the garage with a good tire. Yes, I said tire. Believe it or not, there's a lot of good reading on your tires but the information is presented in code. Solving the "Mystery Of The Tire Code" is bit like being a master detective and master spy all rolled into one, so it can be fun as well as informative.

Apart from the tire manufacturer's name and the tire model, such as "Goodyear Eagle GT," those numbers and letters on the sidewalls of your vehicle's tires will tell you the tire size, the maximum load rating, maximum inflation level, whether the tire is of radial or bias design, and whether it is a tube or tubeless tire. You'll also find out the tire ply composition and materials used, the Department of Transportation (DOT) safety code, treadwear, traction and temperature grade, and get a safety warning. Tire manufacturers put this information on every car and truck tire they manufacture and the markings have been standardized under the Uniform Tire Quality Grading (UTQG) system, so all brands and types of tires are marked in the same way.

If you own a new car or have always let the local professional tire dealer put tires on your vehicle, you may not think it is important to be able to read a tire -- and you might be right. Tires generally last a long time and you could be into your next car by the time your current treads need replacing. On the other hand, you could be driving away from home and get a blowout. If this happens, you need to be sure the tire you are buying is the right one for your vehicle.

And suppose you've just purchased a used car or truck. How do you know the previous owner has installed the correct tires on the vehicle? My neighbor recently bought a used sports car that should have had tires rated for 130 mph. However, after reading the sidewalls, we realized the tires were actually speed rated for only 112 mph. Even though my friend doesn't drive fast enough to need tires rated at 130 mph, I pointed out that his car came from the factory with tires rated for the higher speed for a reason. The vehicle is designed to handle best with those tires and installing the correct rubber will increase the margin of safety.

Reading the sidewall

A typical tire sidewall 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 as original equipment on some light trucks). The "205" is the nominal 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 tire's load rating. It tells you the maximum weight the tire can carry. A load rating of "89" means the tire is rated at a maximum of 1279 pounds. The "H" is the vehicle's speed rating (up to 130 mph).

An "LT" instead of a "P" means the tire is rated for use on light trucks (pickups, vans, sport utilities) and a "B" or "D" in place of the "R" signifies the tire is belted bias or diagonal bias construction, respectively. Virtually all passenger and light truck tires installed as original equipment these days are radials, but many trailer and specialized use tires still use bias or diagonal bias construction.

Treadwear

A tire's treadwear rating, printed on the sidewall as a three-digit number, is a comparative rating designed to give you an idea of the expected tread life of a tire. The base treadwear rating for passenger tires is 100, which translates into 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. So a tire with a treadwear rating of 150, for example, means it should have a 50 percent longer tread life, or 15,000 additional miles over a base tire rating. But don't make the mistake of thinking that you'll actually get that mileage out of the tire. Treadwear ratings for a given tire are determined by controlled test conditions on a racetrack and your tire wear will vary, depending upon the actual driving conditions, maintenance, climate, road type, and driving habits.

Traction rating

A tire's ability to grip the road is rated alphabetically as either "AA," "A," "B" or "C," with "AA" being the highest. These letters represent the tire's ability to stop on wet pavement (asphalt and concrete) as measured under controlled conditions on a test track. A tire that is given "AA" traction rating has superior wet braking traction (this rating is usually reserved for specially-designed rain tires) over standard tires. An "A" grade signifies that the tire has excellent wet braking traction on concrete and asphalt, "B" is the middle performance standard (average), and "C" is the lowest traction grade. Don't make the mistake of thinking that a "B" or a "C" grade means a tire has poor traction. The alphabetic rating is only a measurement of a vehicle's traction on wet pavement and is for comparison purposes only. 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 the Arizona desert, for example, a C-rated tire may provide excellent traction for weather and road conditions that are usually dry. If you live in the rainy Pacific Northwest, however, you may want to consider a tire that has an "A" or "AA" rating for maximum gripping power during inclement conditions.

Speed rating

Passenger tires will be marked with one of several speed symbols, for example "S", "T", "H", "V", or "Z." These symbols indicate a tire's speed capability based upon laboratory tests under ideal conditions and aren't valid if the tire is underinflated, worn out, damaged, or overloaded. The following list shows each tire speed symbol with its equivalent speed rating:

"S" -- maximum speed of up to 112 mph.
"T" -- maximum speed of up to 118 mph.
"H" -- maximum speed of up to 130 mph.
"V" -- maximum speed of up to 149 mph.
"Z" -- over 149 mph.

Most compact economy cars and sedans have tires rated at "S" or "T." Performance sedans and sports cars have tires rated "H," "V," or "Z," depending upon the vehicle's capability. It is important to remember, however, that just because a tire is rated at a certain speed, it doesn't mean you can drive the vehicle safely at that maximum speed. A sports car outfitted with "Z" rated tires, for example, cannot be safely driven on most U.S. highways at the speed for which the tire is rated because the roads aren't in good enough condition. Additionally, traffic volume, weather conditions, vehicle condition, driver skill and a maximum speed limit of 70 mph on most freeways, preclude such folly -- not to mention the ever-vigilant state highway patrol!

Load rating

The load rating is the amount of weight a tire can safely carry. Passenger car tires are rated for their load carrying ability with a numeric designation ranging from 65 (639 lbs.) to 104 (1,984 lbs.), with most passenger tires rated between 75 and 100. Deciphering a specific load carrying ability by reading this code isn't all that important as the maximum load rating, in pounds 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 from passenger tires and may not have a load index or speed rating in the size designation, as it isn't required. Instead, you may see letters like "M+S" (mud and snow) or A/T (all-terrain) that 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. As for load index, it is listed alphabetically, from "A" to "E," with "E" being highest. Generally, the lighter the intended use of the truck, the lighter the index rating of its tires. A two-wheel drive compact pickup may have "A" rated tires because it isn't designed as a vehicle that regularly carries heavy loads. A full-size Suburban with four-wheel drive and a Class IV hitch, on the other hand, is designed to haul heavy loads and will probably have tires that carry an "E" rating.

Making sense of all the numbers and letters stamped on the sidewall of a tire can be a bit daunting at first, but pays to have a better understanding of your tire's intended use and capabilities. If you'd like to learn more about tire size, tire ratings, tire construction, tire technology or tire care, click here to go to iCARumba's Car Care Encyclopedia.