Castle Creations motor and ESC combo recommendations for the Traxxas Maxx.

Monster X w/1512-1800kv - 25 pinion / 50 spur on 3s = ~30MPH
Sidewinder 8th w/1515-2200kv - 22 pinion / 54 spur on 4s = ~40MPH
Monster X w/1515-2200kv - 24 pinion / 46 spur on 4s = ~50MPH

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.