Most electric cars use between 200 watt hours per mile and 500 watt hours per mile, the Leaf is around 330, pickuptrucks are higher, 1,400 pound two seaters like mine are down at the bottom at 200-250 watt hours per mile and that is measured at the wall going in to the charger.
So in theory, a
Honda eu2000i gas engine generator says it can run on 1.1 gallons for 4 hours putting out 1,600 watts, that is about right for my 1.5kw charger and that charger is new enough to me that I haven't fully tested it yet, but I hear from others that an hour of charging will give me around 8 miles of range, so if I charged with a gas generator I'd be getting 29mpg, not very good, but it could be done and after 4-5 hours my battery pack would be full.
Just to be clear about how the Volt really works, yes the gas engine can charge the batteries, but that is the least efficient way that it can work and it rarely happens, the transmission is more like a three way split with planetary gears, very close to what the Toyota Prius does, but just different enough that they have their own patents, there for the gas engine can turn the wheel, the motor can turn the wheels and the gas engine can turn the motor to work as a generator, when it mouton mode both the gas engine and electric motor are working together to pull the 4,000 pound car up the long mouton pass, as I understand it the rest of the time it's 100% electric until the battery is down to 20% then it's 100% gas with electric regen braking.