Originally Posted by Myosmith

Rolling resistance is only part of the equation. As far as physical quality of the tire, weight, tread compound, suppleness, and aerodynamics also need to be considered. You still don't have the full story as there are rider factors that come into play. When I started out 23mm on narrow rims was all the rage, even for Clydesdales who had to pump them up rock hard to prevent snakebite. I hated the ride and the feel but put up with them because they were "faster". Then I tried Bontrager AW3 lites in 25mm and found that my speed actually increased on medium to long rides through the hills of MN lakes country. Part of it might have been rolling resistance, aerodynamics, etc, but IMHO the more important factor is that the tires were more supple and didn't have to be inflated to rock hard. The net result was that I felt more comfortable for longer and was more confident on corners and less than perfect road surfaces, so I rode faster and had less fatigue. Now I ride 25mm on a moderately wide rim on my road bike and 28mm in the same tire on a slightly wider rim on my mutt. Way more fun, very comfortable and hasn't hurt my overall speed at all. I might lose a minutia of speed on climbs but make up for it on descents and corners, and (added bonus) at the end of the day my hands aren't numb from road buzz.