Treadwear Grades?

Need to purchase new tires for the wifes volvo. The pirellis that came on it only lasted 22k miles...most owners of the xc90 say they are only getting 20-30k out of a set of tires, no matter the brand. My question is how important is the Treadwear, Traction and Temp ratings of a tire. The tire manufacture assigns these grades and it isnt governed so seems there could be some bias. Also I found a continental tire (which I hated on my 4runner) that has a treadwear rating of 540 (way higher than any other I have found) and a treadlife warranty of 6 years/50k miles.

 

Treadwear grades are based on some kind of testing where a number of vehicles are driven on a test course and tread depth is measured about every 800 miles. Vehicle usually driven about 8,000 miles during that time.

The higher the number, the longer the tire is supposed to last. 100 is kind of a base point, it's a "regular" tire. A treadwear rating of 200 means the tire should twice as long, one thats 300 means three times as long, etc...540 would be a solid rating.

Rotating the tires is very important. Front tires typically wear out faster than rear tires, if you're not rotating. Not only are the front tires rotating, they are also being turned so it creates a little more wear on the inner/outer edgs of the tire.