Because the 65cc motor with its 160kv will only turn 6700rpms max on 12 cells(26-15prop). This limits top speed to 95-100mph if it unloads all the way. The 50cc motor has a 230kv and will turn 8500rpm (22-12 prop). I checked speed this last weekend using an onboard camera recording GPS speed and got top speeds of 110+ mph with a peak on one run ending in a vertical climb of 114mph. Since I don't do long dives to build speed this means that because the P 51 being a low drag airframe is allowing the prop to operate at slightly better than 100% efficiency It is flying faster than computed prop speed (108mph).