For my Jamis Renegade that I use on gravel, rail trail, towpath type rides, I put on the 35mm Schwalbe Marathons and they are plenty wide/comfortable/durable for those type of surfaces. I've done a few sections that were closer to technical trail riding and both the tread and the width weren't really up to that - at least at my skill level, since I'm 90% a road rider.