Castle Creations motor and ESC combo recommendations for the Losi Tenacity (DB & TT):

Sidewinder 4 w/1412-3200kv - Stock gearing (15/40) on 2s = ~31MPH
Mamba X w/1415-2400kv - 17T pinion / 40T spur on 3s = ~40MPH
Monster X w/1515-2200kv - 17T pinion / 40T spur on 4s = ~53MPH

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.