Ok, I had 4 of hpi ss monster wheels rear with proline gladiator tires already glued and thought I'll try them. I reversed the tranny, put the 28t pinion back on and let her rip,this thing is now a bad off roader. It is a little slower probably high 20s but I think it will handle a 31t pinion fine. I got about the same runtime I was getting with a 25t pinion and the stock tires using a 2s 2200mah lipo which was around 20-25 minutes. The wheel nuts just barely get into the nylon locking part but I did not have one come lose in two packs.
Anybody that needs more ground clearance, should try this, I was running it in our pasture with some tall grass and it was handling it good. The tires actually measured almost 4.2, it looked funny at first but it grows on you and I have change the steering servo to a hs-82mg just for insurance.