Well... it works like a diesel electric locomotive.
The batteries are charged while in the garage to about 90% or so capacity and then you drive till they hit 30% capacity and then the gas motor kicks in to give power to the electric motor while the battery acts as a surplus charge for extreme driving conditions.
It’s ultimately the best of all worlds.
The generator operates at 99% efficiency (most regular autos run theirs at about 20-30%) thus giving it about 50mpg as opposed to 30mpg.
The charge control keeps the battery at optimal cycling(and has a built-in heatsink to regulate battery temp) so it virtually guarantees a 5-10 yr life cycle on the batteries.
And the use of electricity rather than gas makes it more efficient on the whole. That being rather than using x amount of watt-hours to go x miles you use the same amount to go 4-5x.
Really the estimates are that at worst, with a dead Volt battery you’re stuck going 70mph or so and maybe 55mph on a slight incline.
Generator efficiency needs to include the efficiency of the engine turning it.