Castle Creations motor and ESC combo recommendations for the Losi LST 3XL-E.

Monster X w/1515-2200kv - Stock gearing (22T pinion) on 4s = ~30MPH
Cobra 8 w/1515-2200kv - Stock gearing (22T pinion) on 6s = ~45MPH
Monster X 8s w/1520-1650kv - 25T pinion on 8s = ~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.