I'm no expert, but I think given the relatively flat torque curve of electrics that two speeds is all that is really required. The
Killacycle uses a direct drive it seems but "shifts" by switching its two motors from series to parallel, FYI.
On the other end of the spectrum, for my bicycle sized machine I'm thinking something along the lines of
retro-direct, to make sure I can have decent top speed and still climb hills.