A factor that has not been mentioned is the depth of the tire's SIPES. Sipes are the little grooves that typically run diagonally across the tread and look like closed cuts. Most premium tires have sipes that run the full depth of the tread, but many tires have sipes that go only half-depth. Those "trivial-looking" sipes make a difference in wet weather and in dry cornering. So if you have half-depth sipes, then consider replacing the tires when they have more tread compared to tires with full-depth sipes.