As has been said above, you really need to do a bit of research on permanent magnet motor characteristics and understand how current, torque, voltage, rpm and motor efficiency are interlinked.
As a general guide, voltage determines rpm and current determines torque. This means that, wholly unlike an internal combustion engine, you can have a high torque at a very low rpm, and pretty much the same torque over a wide rpm range. PM electric motors tend to be more efficient when run at lower than maximum current, so there is some merit in using variable gearing to keep the motor running within its most efficient operating range.
This changes the way you need to think about gearboxes with variable ratios. For an internal combustion engine the gearbox is essential as a torque multiplier for low speeds, to overcome the lack of torque that these engines produce when run at low rpm. For a PM electric motor the situation is very different, a gearbox is only needed to match the required wheel speed to the rpm range over which the motor will run efficiently. This is quite a wide rpm range in practice, so there is little need for a wide range of variable gear ratios.
