Multi geared boxes usually make up for the loss in efficiency by having better acceleration. 1 gear boxes are rare. With electric and reversible drive 1 speed is all you need for 55mph or less so long as the motor isn't too small. But even electric motors have their peaks. The 2 speed Tesla box was supposed to give them a better top speed. Electric motors produce a lot of torque for their horsepower. Torque is the force that blows gears through transmissions and snaps axles.
Run-of-the-mill 5 Speed manual trannies with OD are 85%-88% efficient. Borg-Warner is probably the largest manufacturer of transmissions. My personal favorite is their T-56
. One speed transmissions are there just to give a better ratio than the final drive can provide.