It's all about low drag and gears if you have low power, on a road cycle a fit person can easily go 45, apply 2x the power of a human to that equation and you should be able to go about 55 factoring in drag. Selectable gears allow you to use the sweet spot of the motor and yourself and really make a huge difference and I recommend them on lower powered builds because they are liberating in a sense that you won't bog hills and will be able to cruise at higher speeds at a lower motor speed.