Methodology
The wear on tires that are being tested ("candidate tires") is compared to the wear of Course Monitoring Tires (CMT), which are sold by the NHTSA at its UTQG test facility in
San Angelo, Texas. Both types of tires are mounted on vehicles that will be driven in a
convoy during the test, thus ensuring that the candidate tires and the CMT tires experience the same road conditions. The convoy, typically one of four or fewer vehicles, will drive 7200 miles on
public roads in
West Texas. Candidate tire wear will be checked during and after the test, and compared to the wear on the CMT tires from the same convoy.
The first CMTs were commercially-available Goodyear Custom Steelguards, and
Goodyear Tire and Rubber Company produced all CMT tires from
1975 until
1984. From
1984 to
1991, the CMT tires were produced by
Uniroyal. CMT tires are now "specially designed and built to
American Society for Testing and Materials (ASTM) standard E1136 to have particularly narrow limits of variability."
1.
Treadwear Grade Number
The
treadwear grade describes how long the tire manufacturer expects the tire to last. A Course Monitoring Tire (the standard tire that a test tire will be compared to) has a rating of "100". If a
manufacturer assigns a treadwear rating of 200 to a new tire, they are indicating that they expect the new tire to have a useful lifespan that is 200% of the life of a Course Monitoring Tire.
Limitations
The DOT does not test tires. It depends on manufacturers to test their own tires and report the results. Unfortunately, this system has made treadwear ratings far less useful than the DOT had originally intended because tire manufacturers are able to use the treadwear grade as a marketing tool.
It is legal and permissible for a manufacturer to give their tire a 240 rating when their competitor's equivalent tire has a 220 rating; thus creating the false impression that the 240 tire is a better purchase because it will last longer. This tendency to inflate treadwear numbers has become so common that some manufacturers may report that ALL their tires have above average treadwear grades. Some are taking normal tires and reporting a treadwear of 600 or more, or giving a 220 rating to maximum performance tires with a reputation for poor tire life (e.g.
the Goodyear Eagle F1 Supercar EMT).
