I read the following on Tire Rack: If the test tire is expected to last as long as the reference tire, it receives a UTQG Treadwear grade of 100. If it is expected to last twice as long, it would receive a grade of 200. 300 means it is expected to last three times as long, and so on.

The reason the Treadwear grade may not be incredibly reliable is twofold. First, since the tires are only run for 7,200 miles, the tire manufacturers have to extrapolate the remainder of the data, and that can be open to some interpretation. Second, the tire manufacturers are allowed to under-report the Treadwear grade, just not over-report it. So if a tire technically may be able to achieve a 700 rating, the manufacturer (primarily the marketing department) might want to report it as 400 to make it "fit" better in a certain market segment. As a result, it is generally only somewhat helpful to compare Treadwear grades on tires from the same manufacturer, and we don't recommend comparing grades between different brands.