One thing about a car, is that the automatic transmission controls the torque applied to the gearbox, in addition to changing the gears. Also, if a car shifts abruptly, or at an unexpected time, the driver doesn't usually lose control of the car. So those are things to think about.

Sram makes an automatic 2-speed. Something simple like that, with maybe 3 speeds and no batteries, might be appropriate in many regions of the world, where cities are pretty flat. Or, the system should recharge its own batteries.