Some people buy tires based on price, assuming that a higher cost means a better tire. Some go by brand name. Some select on the basis of how cool the logo looks. Others just ask for some new ones. Each method may work, but the surest way to get value for your money is to be informed. Use the facts molded into tire sidewalls to make comparisons easier.
Some of what's written on the sidewall, like the brand name, is straightforward. You'll also find the maximum load in pounds that the tire is rated to handle, its construction type ("tubeless steel- belted radial"), and the maximum air pressure (different from the operating pressure) that the tire's designed to handle.
Cracking the codes
* Traction, heat resistance, tread wear Two important indications of quality are how long a tire will last and how well it will grip the road. The Department of Transportation (DOT) requires passenger car tires (except for snow tires) to be graded on three factors bearing on these qualities.
The traction index (AA, A, B, or C) expresses the ability to stop on wet pavement. AA, a relatively new designation, is the best. The C rating means the tire meets the minimum federal standard.
The temperature index (again, A, B, or C) rates ability to dissipate heat, a major tire killer. The top rating is A, while C indicates compliance with the federal minimum requirement.
The tread wear index is a somewhat helpful indication of how well the tire resists wear. The reference point is 100, so a rating of 300 means the tire should last three times as long as one from the same manufacturer rated 100. Each tire maker can select the tire it wants to use as a reference, so this figure is good for comparisons within brands, but not necessarily for comparisons among brands. Further, the figures don't translate to miles.
DOT provides a lot of information, including charts for easy comparisons, at its Web page "Uniform Tire Quality Grading," UTQG.
* Size In a typical size code, such as P185/75R14, the first letter indicates vehicle type—in this case P for passenger car; LT means light truck. The letter code doesn't always appear, however. The first number, 185, is the tire's width in millimeters. The second, 75, is a comparison of height to width. This tire's sidewall measures 75 percent of its width. R gives construction type, radial; nearly all passenger car tires these days are radials. Bias belted (B) is another category. Finally, 14 is the diameter in inches of the wheel the tire fits on. Generally, new tires should be the same size and type as those being replaced.
Sometimes a speed-rating letter precedes the construction type letter. This indicates the maximum speed safe for the tire, but is not an invitation to drive that fast. Examples of speed-rating letters are Q (100 mph), U (124 mph), and Z (more than 149 mph).
* DOT number This combination of numbers and letters identifies, among other things, the manufacturer and the specific factory that produced the tire.
When you buy
Either you or the dealer should fill in the registration form so you'll be notified of any recalls. When you buy tires, have them balanced and get new valve stems.
Keep tires properly inflated (you'll find air pressure recommendations in the car owner's manual or on a sticker in the glove box or doorjamb). Inspect them for wear and have them rotated periodically.
* Inflation is measured in pounds per square inch (psi) of air pressure, determined while the tire is cool—before you've driven more than a mile or so. Underinflation can shorten tire life and reduce gas mileage. Buy a high-quality gauge (those on gas station air hoses get hard use) and check pressure at least monthly.
* Inspect the tires when you check the air pressure or fill the tank. Tread should be at least 1¼16 of an inch, and wear should be even. Uneven wear can indicate improper inflation or tip you off that suspension components may be past their best. Watch also for bulges and cuts on tread or sidewalls.
* Rotate tires to even the wear—this lets you replace all four tires at the same time and prolongs tire life. A typical schedule would be every 6,000 miles, but schedules and patterns vary from car to car, so consult the owner's manual.
This article was first published in November 2003. Some facts may have aged gracelessly. Please call ahead to verify information.