 In theory, this means that a tire with a 200 grade will wear twice as long as a tire with a 100 grade.However, tire manufacturers are not under any obligation to grade a tire based on the test results, except to say that they cannot overstate the grade.This is enforced by NHTSA requiring documentation to justify any assignment of a grade on a tire, "These treadwear grades are no guarantee of actual tire mileage; differences in driving habits, service practices, climate, and road characteristics will affect a tire's longevity."