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Tireglyphics

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On: Thu, Sep 15, 2011 at 5:26PM | By: Peter C Sessler


Tireglyphics

If you’ve ever taken a really close look at your tires, you’d probably noticed lots of letters and numbers on the sidewalls. There’s lots of information there and the best way to explain what they mean is to decode the most important numbers found on a tire. In our example, we’ll use 205/65R 15 89V.

205: This number tells us how wide a tire is from sidewall to sidewall and it is measured in millimeters. In this case, the tire measures 205 millimeters. The larger the number—the wider the tire. I have found that 205 millimeters from one manufacturer doesn’t match the 205 millimeters from another—some 205 tires are wider than others.

65: This number represents the tire’s aspect—the ratio of the tire’s height against its width. In this example, the tire’s sidewall height is 65 percent of its width. The lower the number, the more performance-oriented the tire is because a lower aspect ratio results in better steering response and handling—but it can also mean stiffer ride and poor traction in snow.

R: R stands for Radial construction and it has been the standard since the late 1970s. Previously, tires were of bias-ply construction and certain vehicles still may use bias-ply tires. The letter B is used to designate these tires.

15: This number stands for the wheel rim’s diameter. It used to be that 15 inches was the “big” wheel, but now wheel sizes go all the way up to 22 inches. Usually, larger rims are often matched with very low aspect ratio tires thereby maintaining a reasonable tire/wheel height combination.

89: This number indicates the tire’s load index, which measures how much weight a tire can handle. In this case, 89 means 1,260 lbs. (other numbers are listed in the Maximum Load-Carrying Capacity Per Tire chart found at the tire dealer) If you multiply this number by four, you’ll have the total capacity of a set of tires.

V: This letter indicates the speed rating of the tire—which is defined as the maximum speed a tire is designed to run for an extended period of time. In this case, V stands for 149 mph. If you exceed this speed you run the risk of the tire overheating and failing. Other speed ratings are: S 112 mph, T 118 mph, U 124 mph, H 130 mph, V 149 mph, W 168 mph, Y 186 mph. Tires that are rated greater than 149 mph typically have the ZR designation. Not all tires have these speed ratings—as most passenger car tires don’t—meaning that they aren’t rated for sustained speeds greater than 112 mph.

You’ll also find DOT (Department of Transportation) numbers on tires. These are used to track the tires should a recall be necessary. There’s also a traction rating letter—A, B, or C. This is a relative rating, meaning when a tire is compared with another tire from the same manufacturer—and not against a comparable tire from another manufacturer. The traction rating measures straight line braking on a wet road. A is the best and C is the worst. Similarly, there is a temperature rating: A, B, or C, with A being best and C being the worst.

There’s also a treadwear rating on the tire’s sidewall. A high performance tire that is made from a softer tread compound may have a treadwear rating of 150-200—and might last 10,000 miles. The higher the number, the longer it is supposed to last and the current treadwear rating ranges from 20 to 620.




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