My son’s 9 volt RF Monster Truck he got from Radio Shack is more fuel efficient - and you won’t see one of those on the freeway either.
Whoops, 16 kWH is the capacity of the battery, but they only charge up to 80% and discharge down to 30% capacities to extend battery life.
http://www.caranddriver.com/news/2011-chevrolet-volt-first-drive-car-news
So make that question, what do you pay for 8 kWH?