It doesn't. Not only does it not make sense to use food to produce fuel, unless you're feeding a horse, but the energy payout on ethanol versus gasoline ends any actual energy savings. Oh, there's the noise about ehtanol by cellulose fermentation, but it's like thermonuclear fusion reactors: nobody's been able to develop a cost-effective process that produces more energy than it takes in. Truth be told, you're better off riding a bike, unless you have a source for methane production that doesn't involve regular trips to Taco Bell.