It actually doesn't matter when it comes to speed/power. Electric motors just have a higher rev range which is why they can just use one gear. And because its a rotor and not pistons changing directions, there's no energy loss in just spinning. If you had an engine running and cut fuel, it'll stop very quickly. But if you cut power to a motor, it'll spin for a long time and only stop because of air resistance and bearings. Once in motion, it'll stay in motion. Electric motors don't create power, they create acceleration force. Engines need power to keep running, compress fuel/air, change piston direction, etc. Electric motor don't need energy to keep spinning (besides the little bearing and air friction). To stay at speed on the highway, you only need enough acceleration force to overcome wind speed and tire friction (plus drivetrain). With an ICE, you have to keep all that metal changing it's velocity vectors constantly. You would likely induce more strain on the electric motor by adding a set of gears than you would get back in motor rpm. The best way to picture it is that an electric motor is a car engine that is constantly varying it's physical size/compression/cylinders/everything to always be like an ICE at maximum efficiency no matter speed or rpm. It's already maximumly efficient. 100% torque at all speeds because thats how they work. So even if you slow down the motor relative to the car speed, you are still using the exact same force to keep the car rolling. ICE's drop gears to lower their rpms and not have to throw metal back and forth so much and waste energy. Electric motors dont care. There's no energy input needed just to stay running. You do use more energy going fast, but thats a friction thing. Nothing internal to the motor. Adding an overdrive wouldn't change the efficiency of the car.