The Energy Department says with ethanol and other oxygenates comprising 10 percent of the fuel supply for cars and light trucks, the energy content of gasoline has fallen by 3 percent over a 20-year period. A 1990 law requires use of oxygenates to produce a cleaner-burning fuel. “While these additives reduced air pollution, they also resulted in lower heating value compared with conventional gasoline, translating to fewer miles per gallon, because they have lower energy density,” said the department.