I'm no expert on tires, but I believe summer tires wear much faster than all-season tires. I switched out my Bridgestone Alenzas for Conti DWS06 on my 19" rims as soon as I got my X3 M40i. I'm happy and surprised to report that after 2 years and 25,000 miles of aggressive driving, they still have plenty of tread left. I bet they'll last another 10,000 miles. They are great tires.
