From what I've read, that wouldn't be relevant for an electric vehicle, since there is no transmission. Or that might only be for the Tesla.
Electric motors have no need for one, because they have both variable speeds and generally tremendous amounts of torque to start with.
Scroll 2/3 of the way down this page to see a representative brushless DC motor performance curve, for example: http://www.johnsonelectric.com/en/resources-for-engineers/au...
There would be some minor efficiency gains from having multiple gears, but it's probably small enough that it would be hard to overcome the extra losses from having a more complex transmission.
Note that technically electric cars do have a transmission. The motor wants to spin much faster than the tires, so you want a reduction gear to let them both live in the ranges where they're happy. It's a single-gear transmission, though, on every electric car I'm aware of.