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I think the limitation for speed in any of the lower gears is magnetic saturation - at some point, the motor just can't get any more magnetic, and spin any faster or harder.
In the higher gears, how many amps you can pull from the batteries limits how fast you can go. Just because I can go 65mph, doesn't mean I want to, because it drains the batteries SO fast. Also, my main fuse right now is a slow-blow 300 amp, so I don't want to sustain anything over 300 for anything other than brief accelerating.
Upshifting puts more load on the motor, allowing it to draw more amps, allowing greater acceleration.
Driving in a lower gear at higher RPM draws nice low current (good for the batteries!) I would imagine that it also makes increased brush wear.
The other thing is that the motor does seem to have a "sweet-spot". There is a point in the speed of the car and the RPM of the motor where is seems to perform really well, and not pull as much amperage as some other points of the RPM/Speed/Gear curve.
However, since I have no tachometer and having been playing with different system voltages, I couldn't say exactly what it is.
I may just try driving around using just third for a while and see how that goes.
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