Are you sure an electric car can get 230 mpg?

Maybe I shouldn't be nitpicking late night comedy, but here goes: yesterday, in his monologue in the Tonight Show, Conan O'Brien incredulously quoted the news that GM says the electric Chevy Volt can get 230 miles per gallon. He contacted a GM spokesman "by satellite" who said the Volt can achieve that mpg if going downhill, with a sail, on a car transport truck which in turn is on a conveyor belt. Funny ha ha. But my question is: if the Volt is an electric car, what is the substance of which a gallon makes it go for 230 miles?