I think I'm supposed to say somethign sarcastic here, but hey, if a car get get better gas mileage, I'm all for it.
Yeah...but it only goes 5 MPH.
It doesn't get better gas mileage. Think about it. He charges the batteries from his outlet. He's not get the extra miles from the gasoline, he's getting them from the natural gas, coal, protons, etc. that is energizing his house. Most certainly at less efficiency.
By this logic one could push their car to work, drive it home, and thus double their mileage.