500 watt motor, 20x4 inch fat tires and a 15 amp controller, none of that adds up to speed. To quote Jaws, " I think you're going to need a bigger boat".
dont mean to be snarky,your bike wont hit 50 mph coasting ( in full aero profile?) ?get a Surron or one of those soon to be banned ecycles with pedals high speeds are pretty dangerous on a bike.