If you mean the well known Graupner Speed 900s at 6s, 4:1 and 100mm wheels, you're looking at a theoretical top speed of almost 40mph. I'd gear down a lot more. In most arenas you'll never really see more than 15mph, 20mph from one end of the heavy arenas to the other, perhaps.

If these are slower revving motors, say 4000rpm @12v or something, you're still looking at nearly 25mph at 6s.