It had an ICE-powered generator that could also be connected via clutches directly to the drivetrain for better efficiency at highway speeds.
ICE -> Generator -> Inverter -> Electric Motor is worse for efficiency at constant speed on the highway than just connecting the ICE directly to the wheels.
OTOH going the generator route allows you to avoid quite a bit of drivetrain - a transmission, differential(s), etc. Wouldn't surprise me if actual efficiency comes quite close considering you shed 200lbs or so of weight. More than that if AWD.
ICE -> Generator -> Inverter -> Electric Motor is worse for efficiency at constant speed on the highway than just connecting the ICE directly to the wheels.
https://www.plugincars.com/exclusive-chevrolet-volt-chief-en...