Tire Defects and Failures
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Tire Markings and Tire Size Markings Guide
U.S. Tire Markings
Below are the tire markings most commonly seen on tires sold in the United States. All tires must have a U.S. Department of Transportation (DOT) approval, which indicates that the tire has passed all minimum DOT standards.
Tire Size Markings
The tire size shown below is 185/60R14. The "185" represents the tire's section width (tire width in mm). "60" is the tire's aspect ratio, or the ratio of the sidewall height to the tread width. The "R" represents tire construction, in this case radial. The last item is the "14" which represents the rim/wheel size.
Uniform Tire Quality Grading (UTQG) Marking
The Department of Transportation requires each manufacturer to grade its tires under the Uniform Tire Quality Grade (UTQG) labeling system which establishes ratings for treadwear, traction, and temperature resistance. These test are conducted independently by each manufacturer following government guidelines to assign values that represent a comparison between the tested tire and a control tire. While traction and temperature resistance ratings are specific performance levels, the treadwear ratings are assigned by manufacturers following field testing and are most accurate when comparing tires of the same brand.
Tire Size and Tire Markings Further Explained
The tire size is written on the tire's sidewall. Here's more information on the way tire sizing looks on the sidewall:
- P - indicates a passenger (car-type) tire. Other options would be no P indicating metric sizing (essentially the same as P-sizing, which has its heritage in Europe) or LT for light truck. LT truck tires are designed for heavier loads and more rugged service conditions.
- Width - of the tire, in millimeters. The higher the number the wider the tire.
- Aspect Ratio - the height of the sidewall section compared to the width of the tire. If this number was a 50, then the tire section is half as tall as it is wide. Short sidewalls deliver crisp handling. Tall sidewalls give a smoother ride. For a specific tire width, the smaller this number, the shorter the sidewall.
- Construction - R indicates radial construction. Unless you specify the other option, bias (which would have a D [diagonal] or B [belted bias] in this position instead of an R), you are purchasing a radial tire.
- Wheel Size Designation - in simple terms, this indicates the distance across the "doughnut hole" of a tire. You must match wheel diameter and tire diameter. For example, a 15-inch diameter tire must only be mounted on a 15-inch diameter rim. A tire with a 15 size designation will not work on a 14-inch wheel, and vice versa. Improperly matching of wheel and tire size can cause serious injury or death during installation.
- Service Description - includes the load index (a number) and a speed symbol (a letter). For load index, remember to keep the number the same as, or greater than, the original tire's load index. That number stands for a specific weight-carrying capability. The higher the number, the greater load-carrying capacity. It is important to keep the number the same or higher, because your tires are matched to the weight of your car. Never choose a tire with a lower load-carrying capacity than the original equipment tire size.
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