Schwalbe Smart Sams (more of a trail tire than a knobby) 700c x 40mm @ about 50 psi. I weight about 230 pounds and the bike about 28. I could do OK until I got to the point that my back tire would spin, then momentum would drop to zero and it was game over. I tried keeping my weight centered over the BB keeping just enough weight on the seat to keep the back tire hooked up. If I moved further forward or stood up with all my weight off the seat, I just spun out sooner. I think it's time to look for a real MTB or at least a monster cross bike that will take some beefier tires.