I don't know what kind of mounting hubs you're using, but I've seen guys run pneumatic go-kart rear slicks and wheels on the rear of their q-scale drag cars. They're a bit heavy, but they look good and last forever.

For the fronts, try narrowing a set of tenth scale T-Maxx 40-series (approx 4" diameter) monster truck wheels and putting a ring of hard foam or rubber on them.

I used FG Marder 1/5 offroad 5-spoke buggy wheels, narrowed on my q-scale funny. You've seen it on QSL.

(on the 'bay, RD Logics aluminum wheels, about the right size for a Q-scale car: item number 190283214378)