If it ain't broke, don't fix it. That sums up the mentality of most replacement-tire buyers. But there are downsides to conventional all-season tires. Cold weather, for instance, can stiffen them so much that they struggle to keep their grip. And besides, specialty tires just make sense sometimes. A run-flat, for example, is a great insurance policy if you often travel through remote areas. And if you have a souped-up sports car, why not run it on the most responsive rubber you can buy? Here are the pros and cons of each of the four major tire types.