"The US Department of Energy has introduced a new website to help consumers compare the driving cost for an electric vehicle (or hybrid in electric mode) versus a conventional gasoline vehicle. "The eGallon represents the cost of driving an electric vehicle (EV) the same distance a gasoline-powered vehicle could travel on one (1) gallon of gasoline."
http://energy.gov/articles/egallon-how-much-cheaper-it-drive-electricity
http://energy.gov/articles/egallon-how-much-cheaper-it-drive-electricity