In a car or truck, petroleum diesel is currently more energy efficient than biodiesel, but things are changing. According to the U.S. Department of Energy, the energy content of a gallon of diesel is 11 percent more than the energy content of a gallon of biodiesel. With all things being equal, a truck running on a mixture of 20 percent biodiesel and 80 percent diesel gets 2.2 miles fewer miles per gallon than a truck running on just diesel [source: U.S. Department of Energy]. Nevertheless, biodiesel is more energy efficient than gasoline.
In addition, producing biodiesel is becoming easier and more energy efficient. How so? Researchers at the University of Idaho and the U.S. Department of Agriculture say for every unit of fossil fuel energy needed to grow and refine soybeans into biodiesel, four-and-a-half units of energy are gained. In comparison, for every unit of fossil fuel needed to produce petroleum diesel, the return is less than one. Researchers say that farmers and refineries are using less fossil fuel and better production methods to turn soybeans into energy-efficient biodiesel [source: U.S. Department of Agriculture].