Castle Creations motor and ESC combo recommendations for the Arrma Infraction 6s V2 (1/7th):

Cobra 6 w/1520-1650kv - 16 pinion/46 spur on 8s = ~70MPH
Cobra 6 w/1717-1650kv - 17 pinion/46 spur on 8s = ~75MPH

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.