Depends on where you go. On this board, 15" are the rule. On others, 19-20" are the big deal. Based on what I've seen and read, actual performance gains start to go away once you get outside of 17" wheel sizes because of the additional weight you are now carrying. Performance wise, a 15" tire can match a 17" tire in grip, but you have to dig to find them and they cost as much as a 17" tire will run you. These aren't donuts you find off the shelf at Tire Rack or Summit, whereas you can get a decent 17" tire almost anywere.
