The knobs really help in mud, but I find that wide slicks work fine on gravel and dry dirt. I also rode the GAP and C&O Canal trails this summer on 32 mm road tires, Panaracer Paselas, which have a little bit of tread. They were fine on the GAP, which is all gravel, except for the steeper downhill portion from the Eastern Continental Divide to Cumberland, MD. The Paselas were a little sketchy in spots where the gravel was thick, but I'm not sure treads would have helped much. However, the C&O had a lot of muddy spots and some CX tires with more tread would have definitely helped there. If I ride the C&O again, my bike will have CX tires.