In real life, the cost of driving a plug-in hybrid like the Volt would depend entirely on how far you drive it between charges and how much you pay for gasoline and electricity.This is a little more scientific than the explanation developed by comedy writers for Conan O’Brian, who noted that the mpg required going downhill, with a sail, etc etc.
If you drive 20 miles on electricity alone, technically your mpg is infinite. If you drive 20 miles on gasoline alone, you might get 30 mpg or more.
Clearly the EPA guidelines need to be revised to give more realistic numbers for plug-in hybrids. But I imagine GM (aka Government Motors) got what they wanted: free publicity for their new (still vaporware) car.