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

Sidewinder 8th w/1515-2200kv - Stock gearing (13/52) on 4s = ~37MPH
Mamba Monster 2 w/1515-2200kv - 16 pinion/52 spur on 4s = ~45MPH
Monster X w/1515-2200kv - Stock gearing (13/52) on 6s = ~55MPH

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.