The system is conceptually based on WWI submarines, which ran on batteries with a back-up gas engine. If you're running low on electrical charge you can run an internal combustion engine, but this is pretty much just a generator and not part of the drivetrain. A full 12-gallon tank of gas will let you go about 640 miles, which means that even if you're burning petrol, you're getting 53 miles per gallon.
I think this will be a great excuse to start building nuclear plants ... tell fucking OPEC that we have a "plan B" for running our cars! As much as I hate driving, this is something that I want.