An automatic transmission bicycle including a bicycle frame, a chain ring, a pair of pedals, a wheel, and a plurality of gears coupled to the bicycle frame. The plurality of gears has a plurality of gear combinations defined by at least two gears cooperatively engaged. The automatic transmission bicycle includes an electric motor coupled to the bicycle frame. The automatic transmission bicycle includes a voltage generator configured to generate a current that has a corresponding voltage dependent upon rotational speed of the wheel. The automatic transmission bicycle includes a gear shifter in electrical communication with the voltage generator and configured to receive the current generated by the voltage generator. The gear shifter shifts between two respective gear combinations from the plurality of gear combinations in response to receiving the current that has a corresponding voltage within a predetermined voltage range.