A tire with a treadwear rating of 320 should last for approximately 40,000 miles. This is a lower-middle level treadwear rating. Tires with a high-end rating of over 500 should deliver approximately 80,000 miles.
The treadwear rating is based on an evaluation of how quickly a tire wears out under controlled conditions on a government approved test course. It is important to remember, however, that a tire's potential performance should also be evaluated on other factors rather than just its readwear rating. For example, speed rating, temperature rating and traction need consideration before deciding whether a particular tire meets the requirements.