I switched out my puny little anorexic 28mm GP40000 II's, which were a measly 30mm on my rims, and put on some Compass Barlow Pass 38mm and pumped them up real good to 35 psi in the front, 40 psi in the back. They "feel" a wee bit more sluggish, but my GPS data say otherwise. (Jan says people sometimes subjectively interpret a harsher ride as a faster ride.)