Castle Creations motor and ESC combo recommendations for the Arrma Big Rock.

Copperhead 10 w/1412-3200kv - Stock gearing (15/57) on 3s = ~50MPH
Mamba X w/1412-2100kv - Stock gearing (20/57) on 4s = ~57 MPH
Mamba X w/1415-2400kv - 18 pinion/57 spur on 4s = ~64MPH

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.