Originally Posted by
NOS88
It's not so much the 700c wheels as it is the width of the tires on it. Readily available tire widths for this size wheel range from 20mm up to 37mm. In gravel you would want something, I would think, in the 30mm to 37mm range. Knobby tires are really designed to grab hold of the surface for traction. I'm not sure of the kind of gravel you'll be on, but for the bike paths around my area with gravel the knobby tires are really needed.
I find that 700 x 25's work pretty well in the gravel/sand surfaced roads arounf here - not deep sand or gravel, but surface sand and gravel. I find a significant difference between 25's, 23's an 20's or 19's, with 25's being noticeably superior.
I wouldn't, however, want to do a long ride on the 25's in sand or gravel - I use them for short distances, such as a gravel trail between a break in the cement surface. For longer rides in sand/gravel, I will choose my Panaracer slicks on the mtn bike.