I've run 23C slicks (Hutchinson Fusion 2) and they were fast but very harsh. I prefer a 28C or 32C tire since the volume gives a better ride and studies have shown that with the same tread, rolling resistance is actually lower for the fat casing but aero drag is higher, although that is a bigger issue above 25mph. Currently I run 32C Paselas and they actually feel faster than the cheapo Forte 25 C slicks I was running. My son has used the Conti Speed tires on the street for several years and really liked them.