I ride 4 miles of gravel road each way to and from work, and I'm happy with 32s. I'm currently running Marathons (not Plus or Supreme) and in the past have run Race Lites, Kwests, and Panaracers. They've all been fine, except when the gravel road is recently graded at which time there are rocks from pea up to about golf ball sized loosely covering the road. During those times riding that road truly sucks and probably would with anything short of very wide MTB tires. I don't think a crushed rock MUP would be that bad ever.