There is a projected change in density and energy content per gallon, thus leading to a small change in fuel economy. In most cases it should be imperceptible- since the change still falls within the range of normal variations in diesel fuel. While fuel economy decreases slightly, there is not an overall energy loss, since refinery production volume increases to offset the energy density change. There is also expected to be a slight improvement in the cetane number of the diesel fuel resulting from a small decrease in aromatic content due to hydrotreating.
Question and Answer was originally posted at Questions and Answers on the Clean Diesel Fuel Rules (PDF)(135 pp, 888 K, EPA420-B-06-010, July 2006, About PDF)