Castle Creations motor and ESC combo recommendations for the Traxxas TRX-4.

Copperhead 10 w/1406-2280kv - Stock gearing (11/46) on 3s = ~5MPH low gear/~13MPH high gear - CRAWLING ONLY
Mamba X w/1406-2850kv - Stock gearing (11/46) on 3s = ~6.5MPH low gear/~16MPH high gear - Trail Running
Mamba X w/1412-3200kv - Stock gearing (11/46) on 3s = ~7.5MPH low gear/~19MPH high gear - Trail running

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.