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A briefing on tire specifications courtesy of Discount Tire Direct.
The United States government established the Uniform Tire Quality Grading (UTQG) system to assist consumers in their purchase of tires. This system is a relative comparison system, and is not a safety rating, nor a guarantee that a tire will last for a prescribed number of miles. Under the UTQG, manufacturers use three criteria to grade tires: treadwear, traction and temperature. The treadwear grade is a comparative rating based on the wear of a tire when tested carefully under controlled conditions. For example, the useful tread on a tire graded 400 should last twice as long as a tire graded 200. However, another tire manufacturer may grade a comparable design 300, so a grade of 150 would last just half as long under its grading scheme. The key here is to not use one manufacturer's grade versus the other, but instead to compare tire grades within a given brand. Also, keep in mind that actual treadwear performance varies tremendously depending on the tire's real-world use. Driving habits, air pressure maintenance, road conditions, and climate affect tire life as well.
Traction grades represent the tire's ability to stop on wet pavement as measured under controlled conditions on asphalt and concrete test surfaces. As of 1997, the traction grades from highest to lowest are "AA," "A," "B," and "C." A tire graded "AA" may have relatively better traction performance than a tire graded lower, based on straight-ahead braking tests. The grades do not take into consideration the cornering or turning performance of a tire.
Temperature grades represent a tire's resistance to heat and its ability to dissipate heat when tested under controlled laboratory test conditions. The grades from highest to lowest are "A," "B," and "C." The grade "C" corresponds to the minimum performance required by federal safety standard. Therefore, the "A" tire is the coolest running, and even though the "C" tire runs hotter, it does not mean it is unsafe.