The US Government has established the UTQG, the Uniform Tire Quality Grading, to assist consumers in their purchase of tires. Basically it's another tool to be used besides the opinions you gather from trusted friends, mechanics and whatever other sources you may have at your disposal. The key to using this system is to understand that it is a relative comparison system. The UTQG is not a safety rating and not a guarantee that a tire will last for a prescribed number of miles. Under UTQG, manufacturers use three criteria to grade tires: treadwear, traction and temperature. The information is right where you need it when buying the tire:
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 their grading scheme. The lesson learned is to not use one manufacturer's grade versus the other, but instead to compare tire grades within a given brand. Actual treadwear performance can vary tremendously according to the tire's real-world use. Variations in driving habits, service practices (most importantly air pressure maintenance), road conditions and climate affect tire life.
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. The temperature grade is established for a tire that is properly inflated and not overloaded.