Wow. This thread is just chock full of misinformation.
First, there are 3 individual ratings in the UTQG rating. In reverse order:
Temperature rating: A misnomer if ever there was one. The test itself is a version of the speed rating test - and while temperature might play a role, the biggest factor is the presence (or absence) or cap plies. The test criteria is speed at which the tire starts to fail - not the temperature.
So, yes, there will be an almost direct correlation between the UTQG temperature rating and the speed rating simply because the tests are so very similar. Plus there are only 3 ratings - A, B, C
These tests are conducted in a lab and repeatability is extremely high. No difference between tire manufacturers or test facilities should be discernible.
Traction rating: The test for this is conducted on a strip of pavement in Texas. It is a comparison of the wet traction of the tire being rated to a control tire, known as the SRTT (Standard Reference Test Tire) That tire is a Uniroyal tire that is produced by Michelin and certified as being the same regardless of the production run. Needless to say, it is quite expensive.
The problem here is that there is only so much time available at the test facility where this strip of pavement is. To get around this problem, most tire manufacturers conduct correlation tests between this pavement and their own test surfaces - and the problem there is that these correlations have proven not to be so good. There are reversals, which makes the assigning the traction rating a bit of a risk. Please note, this has nothing to do with the brand or manufacturer and has everything to do with the way different rubber compounds interact with different pavements. I tend to think of this as a macrotexrure problem, but I am sure it is more complex than that.
This test is less repeatable and adding the fact that some testing takes place on different surfaces adds to that repeatability problem. And while there may be differences between tire manufacturers because of the surfaces the test is actually conducted on, the government does conduct their own testing to verify that the traction rating is accurate.
So how does the Traction rating relate to the Speed rating? ANY tire can be given a high traction tread compound and get a great UTQG traction rating. However, it doesn't make sense to do so for a pretty generic tire - while it does make sense for a high performance tires (meaning high speed rating).
So you will see some correlation between speed rating and traction rating simply because people who buy high performance tires want good traction, not good treadwear.
Plus, there are only 4 ratings available - AA, A, B, and C.
(Did I mention that there is a technology triangle between treadwear, traction (especially wet traction), and fuel economy? Well, there is, and you can't get high UTQG traction rating AND high UTQG treadwear ratings!)
Treadwear rating: This is the test where all the bunk about manufacturer differences comes from. The test itself is somewhat repeatable. It's a comparison of the SRTT to a candidate tire. The test is run with identical vehicles on the same course at the same time.
The problem here is that the test is relatively short - 7200 miles - and while accurate measurement devices are used, it still is only 10% of some tire's capability. Plus, this grade has some pretty small increments - 20 units. The difference between a 700 rating and a 800 rating is only 12%. It just sounds bigger than it actually is.
Put another way, the actual mileage difference between a 700 rated tire and an 800 rated tire could be less than an oil change interval.
Then there is the problem of assigning the grade. The rules say you can't OVER-state the rating, but you can UNDER-state it. Some tire manufacturers don't like to stick their necks out, so they publish conservative numbers. Others like to stretch it to the max!
Then there is the issue of rating between different models of tires from the same manufacturer. The marketing folks want things to line up nicely, so there are some games that can be played there.
So you CAN compare ratings between tire manufacturers, but you have to be aware that small differences aren't really significant. Clearly there are differences between a 300 rated tire and a 600 rated tire regardless of who the manufacturer is.
And lastly, the actual wear you get out of a tire has a lot to do with your driving route. City driving and short commutes wear tires faster than country driving and long commutes. The difference can be astonishing. I've seen tires get less than 10K, while the same tire can go 100K. Those were both extreme cases, and while the average guy won't see those, the middle ground is pretty broad.
