Couple of things. Higher voltage = more speed. Absolutely, controller must be able to take higher voltage or you let the smoke out. But a controller with a low voltage shut off for 36v won't shut off at the right voltage for a 48v battery. Some can be reprogrammed. Also it takes more amps at the higher speed, more wind resistance. Your controller and battery have to be capable of the higher amps or it might struggle to get to the higher speed.