Castle Creations motor and ESC combo recommendations for the Arrma Infraction 3 1/8th (V3 Mega).

Sidewinder 4 w/1410-3800kv - Stock gearing (15/55) on 2s = ~25MPH
Copperhead 10 w/1412-3200kv - 16 pinion/55 spur on 3s = ~35MPH
Mamba X w/1415-2400kv - 18 pinion/55 spur on 4s = ~40MPH

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.