Diesel fuel performance additives improving engine fuel utilization efficiency have been successfully evaluated in the field. An engine dynamometer was used to measure engine operating efficiency direct, and fleet operator's records were used to calculate changes in over-the-road performance.To provide a precise measure of individual truck performance, dynamometer tests were conducted on each engine, directly measuring power output and fuel consumption. This was done because the fleet records indicate that over-the-road performance, as measured by standard fuel economy methods, is highly seasonal. To reduce over-the-road variability, additional calculations were performed considering weight hauled (cargo and equipment), miles traveled and fuel consumed.The direct engine performance measurements and over-the-road performance are compared. Both independent measurements show significant improvements provided by the two multifunctional additives tested.This report describes test methods, equipment and results from the 18 month, 3.5 million mile fleet test.