For strictly monster truck racing (less than 30 second runs):
Mamba X w/1406-5700kv - Stock gearing (14/56) on 2s = ~25MPH

For general bashing and play:
Copperhead 10 w/1410-3800kv - 11 pinion/56 spur on 3s = ~20MPH
Mamba X w/1412-3200kv - 17 pinion/56 spur on 3s = ~26MPH

Note: All speed estimates are with stock tire diameter, high quality batteries and appropriately sized battery connectors.
Always monitor ESC, motor and battery temperatures on new setups or when changing gearing or batteries.