In The False Hope of Biofuels, James Jordan and James Powell note that some studies show that “it takes more energy to make ethanol than one gets out of it”:
But allowing a net positive energy output of 30,000 British thermal units (Btu) per gallon, it would still take four gallons of ethanol from corn to equal one gallon of gasoline. The United States has 73 million acres of corn cropland. At 350 gallons per acre, the entire U.S. corn crop would make 25.5 billion gallons, equivalent to about 6.3 billion gallons of gasoline. The United States consumes 170 billion gallons of gasoline and diesel fuel annually. Thus the entire U.S. corn crop would supply only 3.7 percent of our auto and truck transport demands. Using the entire 300 million acres of U.S. cropland for corn-based ethanol production would meet about 15 percent of the demand.