Castle Creations motor and ESC combo recommendations for the Redcat 1/6 Shredder:

Monster X w/1512-1800kv - stock gearing (15T Pinion / 49T spur) on 3s = ~35MPH
Monster 2 w/1515-2200kv - 17T Pinion / 49T spur on 3s = ~45MPH
Monster X w/1515-2200kv - stock gearing (15T Pinion / 49T spur) on 4s = ~55MPH

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.