I replaced the fatter OEM tires on my old Trek hybrid with 700x28 Continental Ultra Gatorskins. I thought I gained some speed, I don't know if it was because of the narrower tires or the higher pressures that they can handle. I've read that the Gatorskins are not particularly "fast" because of their construction so maybe I could have gained more speed with different tires, but I haven't had a flat in the 3 seasons that I've been riding them.