A drivetrain for a plug-in hybrid vehicle which uses constant gear ratios, rather than any type of multi-gear transmission (traditional transmission gears or CVT) to power the driven wheels in order to increase drivetrain efficiency. A transmission is not a necessary drivetrain component for vehicles with an electric motor/generator (EMG). The EMG is the only mechanical power source until highway speeds are achieved. At that time, the driver may choose to activate and engage the small internal combustion engine (ICE) and the EMG is usually electrically disconnected so that it does not regeneratively charge the vehicle batteries using the power created by the ICE. Constant gear ratios may be achieved by a mechanical gearing system or by a belt drive system. This drivetrain is only for plug-in hybrid vehicles where the batteries are primarily charged by AC electrical outlet or solar (photovoltaic) cells.