Adding ethanol to gasoline reduces air pollution by reducing the amount of greenhouse gasses in vehicle exhaust. Cars and trucks designed to run on flex fuel can use either ethanol biofuel or traditional gasoline for power. Blending ethanol with gasoline can sometimes increase the fuel’s octane and can help engines run better by cleaning the engine and preventing buildup of deposits. Ninety-eight percent of vehicle fuel sold in the United States is a blend of up to 10% ethanol, called E10.