AutoTalk
Understanding your tires
By John Goepel
Some people buy tires based on price. Some go by brand name, or how cool they look. Others just ask for some new ones. But the surest way to get value for your money is to be informed. Understanding the codes molded into tire sidewalls can make comparisons easier.
Some of what's written on the sidewall, like the brand name, is straightforward. You'll also find the tire's maximum load in pounds, its construction type (e.g. “tubeless steel-belted radial”), and its maximum air pressure (different from the operating pressure).
There are several important indicators of tire quality. Passenger car tires (except snow tires) have visible markings showing how they rated in three Department of Transportation quality tests.
The traction index (AA, A, B or C) rates the tire's stopping ability on wet pavement. The temperature index (A, B or C) rates the tire's ability to dissipate heat—a major tire killer. C is the minimum federal requirement.
The tread wear index indicates 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 really only good for comparisons within brands.
Another code you'll find on your tire is the size code. Let's break down an example: P185/70R13. The first letter indicates vehicle type—for example, P means passenger car, LT means light truck. This letter code doesn't always appear. The first number, 185, is the tire's width in millimeters. The second is a comparison of height to width: This tire's sidewall measures 70% of its width. The next letter gives the construction type, in this case R for radial. The final number, 13, 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 safe speed for the tire—which is not an invitation to drive that fast! Examples of speed-rating letters are Q (100 mph), U (124 mph), and Z (149+ mph).
The last code you may find on your tire is the DOT number. This code's letters identify the specific factory that produced the tire, and the last four numbers show the week and year it was made—for example, “3197” means the 31st week of 1997. Some people prefer to buy tires that have not been on a shelf long-term, so this number can be useful.
For more detailed information on tire codes and comparisons, visit www.safercars.gov/Tires.
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