Cracking the Code

Cracking the Code
![Understanding tires]( get the most from your tires, get to know them first.
We’ve all done it. When it’s time to buy new tires, we simply jot down on a scrap of paper the numbers that are printed on the tires we currently own, walk into the dealer, hand that paper to the friendly guy behind the counter and say, “I want four of these.” Wouldn’t it be nifty, though, if you actually understood what all those numbers mean? If you did, you’d be able to ask for a tire that actually is matched to how you drive.

At, we’re here to help. Your tires’ code is easier to understand than you might think, and once you know what it all means, you’ll be much more savvy shopper.

Tire Code

Using the example above of a typical tire code, the very first letter is the Service Type Rating. P, as in our example, means that the tire meets U.S. passenger standards. Other codes include LT for SUVs, full size vans, and pickups, T for temporary, ST for special trailer use, and C tires for commercial use.

The three digits that follow indicate the overall width of the tire in millimeters once the tire is mounted and inflated—in this case 185mm. The two numbers after the slash are the tire’s aspect ratio, or its height compared to its width. In this example, it shows that the tire’s height is 75% of 185mm.

Following the aspect ratio is another letter that indicates the tire’s construction type. R is the most common and indicates radial construction. D indicates a bias-ply tire, where the tire’s internal body plies crisscross on a diagonal pattern, and B is a belted tire, similar to a bias-ply but with an extra layer of reinforcing belts under the tread. And immediately after the construction code are two digits that indicate the diameter of wheel the tire will fit on, in this case a 14-inch wheel.

The next group of numbers and letters represent the tire’s Service Description Rating, with digits that indicate the tire’s load index, and a letter for its speed rating. Load index is how much weight the tire is rated to carry – the higher the number, the more weight it can handle. In our case, 82 indicates that our tire is rated to handle up to 1047 pounds per tire. The speed rating indicates the stiffness of the tire and how it handles. In our case, S indicates that the tire is rated for speeds up to 112 mph. (You can look up the values for load index and speed ratings on the Web.) And if you see M+S stamped on your tire (like you see here at the beginning of the code), it means that it’s an all-season tire rated for use in mud and snow.

The last three digits are the tire’s Uniform Tire Quality Grading, or UTQG. That number, multiplied by 100 will tell you how many miles your tire is expected to last under normal driving conditions. In this example, the UTQG is found just after the word “Treadwear” and is 360, meaning the tire is rated for roughly 36,000 miles (360 × 100). The last letter (or two letters) are the tire’s traction rating on wet pavement, running on a scale from AA (the best) to C (the poorest traction)—our tire has a traction rating of A–and after that is the tested temperature range, or how well the tire dissipates heat, running on a scale from A through C–the better the tire gets rid of heat buildup, the longer it will hold up.

Now that you know the code, you can better choose a tire to match your driving style and conditions. While the size of tire will be dictated by your vehicle’s specifications, things like speed and traction ratings are up to you. Choose wisely, and your tires will serve you well for many future miles.

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