It may be that the speed is limited by the controller box, my front wheel kit is so it would need to put out a higher frequency to the 3phase motor. A way to tell is if the rear wheel has 6 wires in the multicore lead, 3 for power and 3 for the 'hall effect' sensors
The sensor is in the pedal crank in the frame, wire goes in underneath through the frame from the controller. Not sure how to fool it i'm not tech enough. I was looking at fitting a larger sprocket and belt from 'carbon drive' which would turn slower and give a faster speed before cut-out. But the larger sprocket would catch the rear frame to the wheel :/ crafty gtech! There must be a way. Any one else any ideas??