Castle Creations motor and ESC combo recommendations for the Arrma Outcast 6s (1/8th scale).

Monster X w/1515-2200kv - 12T pinion / 50T spur on 6s = ~55MPH
Monster X 8s w/1520-1650kv - 18T pinion / 50T spur on 6s = ~60MPH
Monster X 8s w/1717-1650kv - 15T pinion / 50T spur on 8s = ~70MPH

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.