[David Hodgson]

The treadwear number isn't just a measure of how soft the tread is, it is a number given to the tyre to indicate how long it will last compared to a base number of 100. At tyre with a TW of 420 should last 4.2 times longer than the TW 100 control tyre. Obviously the tread compound has the biggest influence in this, but a case in point: on my daily hack, I recently changed from the RE-002 in Bridgestone's Potenza sport range, which had a TW of 220, to one of the best in their touring range, the Turanza Serenity Plus, which is touted as outstanding dry and wet weather performance - yada yada.
But get this, the TW number is 600. This is partially due the tread being at least 9mm new instead of the average 8mm. I've never seen a number this high. You would think it would be like driving on marbles, but it grips extremely well, though isn't any where near as direct or precise as the sport tyre.