A treadwear rating chart is a matrix of numerical grades assigned to a tire based on the uniform tire quality grade standards. Tire manufacturers assign a treadwear rating to each tire model, not as an absolute test of strength but as a comparison between tire models for a given brand.
The treadwear rating, also called treadwear grade, is a comparative score of the wear of a tire model based on controlled tests. An actual road test is conducted on the product against a control tire, also called a course monitoring tire, that is authorized by the U.S. Department of Transportation's National Highway Traffic Safety Administration. The tires are connected to vehicles running in a convoy to ensure similar road conditions. The public roads in West Texas provide the grounds for the test across 7,200 miles.
Treadwear ratings of tires are not tested by the U.S. Department of Transportation's National Highway Traffic Safety Administration but rather by the tire manufacturers themselves. The tire maker hires a third-party testing company to conduct tests on their tires against the course monitoring tire.
A tire's tread is the surface area of the rubber tire that comes in contact with the road. Many people refer to the tread as the grooves on the rubber tire, but the correct term for the pattern of grooves is tread pattern.