Limitations
Notice that the treadwear grade is a ratio and not a mileage. This is because there are a lot of factors that determine treadwear rates and most of them are a function of driving conditions and operating environment, and not the tire itself. As a result, actual tire wear will vary considerably within the same tire line. The phrase "Your mileage may vary" applies here. In staying that, 2 tires with the exact same compound, should have a treadwear rating that varies in accordance with tread depth. Ie a 400TW tyre have 20% tread depth added, the tyre SHOULD be rated at 480TW
The assigning of UTQG grades is done solely by the tire manufacturer. In many cases, this has resulted in the UTQG grading system to be more of a marketing tool than was originally intended.
It is legal and permissible for a tire manufacturer to give a particular tire line a lower treadwear grade. For example: If the highest treadwear grade in a manufacturer's line up is 600, then a tire line with a lower treadwear test result might receive a grade of 400, instead of the 480 it could possibly receive.
Also, it is common for tires whose treadwear grade is of little commercial value, such as "DOT" racing tires, to be assigned extremely low values - sometimes even zero.
Read more about this topic: Treadwear Rating
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