The tread wear indicates how long the tread on a tire will last. It is part of the uniform tire quality grading system and is determined by testing the tire on a government track under specific conditions.
The tread wear rating is written on the tire. The higher the rating, the longer the tread and tire will last. A tire with a grade of 100 will last half as long as a tire with a grade of 200. The rating is determined by driving and operating conditions, not by the tire itself.
The tire and a Course Monitoring Tire are mounted on the same convoy to ensure they undergo the same driving conditions during testing. The tire is compared to the CMT for wear during and after the test. Manufacturers must provide documentation to the National Highway Traffic Safety Administration when they grade a tire.