<< ... the "Tread", "Traction" and "Temp" rating. Our current tires are Michelin Pilot HX MXV3A's with 240, A and A, respectively, and V rated. There are some Pirellis (P4000 Super Touring) that my local shop suggested that are 320, A, A, in either V or H. >>
I don't have independent knowledge of this but have been told by different sources that the tread wear rating [above 240 and 320] are not comparable between brands. Each manufacturer sets its own basis for comparison, say 100 equals whatever they define it as, and the numbers like 240 and 320 relate to that standard within their own line of tires. So a 240 tread wear rating for Michelin and 320 Pirelli give you no information regarding which has more or less tread wear life.
Within each brand, however, the lower the number the lower the mileage guarantee and, maybe, the greater the stickyness of the tire.
Rob Carpenter [2p worth, I hope]