John W. from Pittsburgh, PA on 7/13/2015 7:42:20 PM:
I think knobby mountain bike tires slow you down too much, forcing you to pedal and exert way too much on such a long and flat journey. Nor would I recommend really thin racing tires, they'll get gobbled up in the mud and ruts on the C&O. I have a Trek 7.3 FX hybrid and have seemed to found a happy medium that is perfect for the GAP/C&O. I use wider tires than a racing bike, but not as wide and knobby as MTB tires. Safe Travels!