Uniform Tire Quality Grading is the term for three specific ratings applied to tires so that consumers can have standardized, easy to understand comparative data when they are searching for the right tire. That's the concept; the reality is somewhat different. Actually, UTQG ratings are difficult for most people to understand, extremely opaque in their relationship to actual tire performance, and in some ways are barely standardized at all.
Traction grades are based on tests to determine the tire's coefficient of friction on wet asphalt and wet concrete at 40 mph. The tire is given a letter grade depending on the amount of G's the tire can withstand on each surface. The grades are:
AA - Above 0.54G on asphalt and above 0.41G on concrete.
A - Above 0.47G on asphalt and above 0.35G on concrete.
B - Above 0.38G on asphalt and above 0.26G on concrete.
C - Less Than 0.38G on asphalt and 0.26G on concrete.
The problem here is twofold. First, who can remember all that when searching for a tire? Second, the traction test does not evaluate the tire's ability to perform dry braking, dry or wet cornering or hydroplaning resistance. These are rather important qualities as well. To evaluate a tire's traction based solely on wet braking is somewhat oversimplifying actual tire performance. This can be actively misleading to many consumers, who might think that a traction grade of AA covers all types of traction rather than just one. A tire that is graded as A for wet braking might well have better lateral grip than another tire graded AA.
The tests are also done in a lab, making it possible to gather much more empirical data, but also calling into question the exact application of that data to real-world conditions.
Temperature grading is based on the tire's ability to dissipate heat while running at high speed against a rotating cylinder. A tire that cannot dissipate heat effectively will break down faster at higher speeds. An A rating means that the tire is able to run for long periods of time at speeds over 155 miles per hour. A B rating means that the tire ran between 100 and 155 miles per hour sustained. A C rating means between 85 and 100 miles per hour sustained. All UTQG-rated tires must be able to effectively run at at least 85 mph.
This can be rather difficult information to process. Do you actually need a tire to function reliably at 115 miles per hour for long periods of time on US highways, or would just 100 mph be good enough? Does extremely good heat dissipation ability have a positive effect on treadwear breakdown even at lower sustained speeds? What is that effect? UTQG temperature ratings simply don't have those answers, and those are the answers people really need to make informed decisions. I'm not even entirely certain of the essential difference between temperature ratings and speed ratings, which also measure the general ability of the tire's structure, such as belts and plies, to hold up under Ludicrous Speed.
Treadwear is perhaps the most complex and least reliable of the UTQG grades. Treadwear grade is tested by running a control tire around a circular track for 7,200 miles, then running the tire to be graded around the same circular track for the same mileage. The treadwear is then extrapolated from this data and compared to a similar extrapolation for the control tire. A grade of 100 means that the tread life is equal to the control tire, whereas a grade of 200 would be twice the treadwear of the control tire. 400 would indicate four times the treadwear of the control, and so on.
The problems here are numerous. The number of actual miles expected of the control tire is not readily available to consumers, so the comparison between it and a consumer tire is simply proportional rather than numerical. Extrapolating the amount of wear after 7,200 miles to determine the actual treadlife over tens of thousands of miles leaves a great deal of room for error and comparing two such extrapolations to each other compounds the problem. Also, it is the tire maker that performs the extrapolation according to their own data model. Since no two tire companies' data models are exactly alike, there can be no standardized result, making comparisons between tires by the same maker only marginal useful, and comparisons of different makes of tires nearly useless. Eugene Peterson, the Tire Program Manager at Consumer Reports, told me once that both the best and worst tread life he had ever seen were tires with the same treadwear rating.
In essence, it seems that UTQG ratings, in a laudable attempt to provide some very simple comparison points, are kind of oversimplified in some ways, and in some other ways are far too complex. The overall effect is that they do not really provide decent comparisons, especially across different makes of tires. Although they can be somewhat useful as part of a comparison of the many different factors that define the quality of tires, one should really take them with a large grain of salt.