[stetrain]

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.

https://www.plugincars.com/exclusive-chevrolet-volt-chief-en...

[TylerE]

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.