Jan Heine of Bicycle Quarterly and Compass Cycles did some testing on this and found that here was very little, if any, speed loss due to rolling resistance with fatter tires. I can’t remember how large they tested but I think it exceeded 42mm. They also found an increase in speed on rougher roads with larger tires due to the smoothing effect. I use 38mm Compass Barlow Pass Extralight tires on my Rivendell Hillborne and was amazed at how fast and smooth a fatter, supple tire can be. This was in comparison to my Bianchi Veloce with 23mm tires. When I realized I was just as fast or faster on the Hilborne, I sold the Bianchi. It was a nice bike but I just didn’t ride it anymore after buying the Riv.

john