[aerohead]

The more we know about your car, and 'how' you'll be driving it, the better we can consider possible options.
There will be a crossover speed at which aerodynamics will come to dominate the road load the motor and battery will be required to overcome.
Motor and transmission efficiency is, for all intents and purposes, fixed.
All you've got is rolling resistance and aero to work with.
The lowest rolling resistance is your friend, as you're thinking.
Your driving speed will tell us something about aero.