This paper describes the physical properties and chemical components of a new fuel additive package along with various testing procedures used in the development of the product. It describes the use of the Fuel Economy Engine Oil Test procedure in determining the relative fuel economy effects of the additive. A modified FEEO procedure is used to screen various components used in the final additive formulation. Another modification of the FEEO procedure is used to determine the amount of time needed to condition the engine to the effects of the product. The effects of the additive on the fuel economy and mechanical thermal efficiency of a General Motors EMD 16-645-CE6 engine burning No. 2D diesel fuel are also described.Other information in the paper includes the effects of the additive system on the chemical and physical properties of fuel especially as outlined by Federal Specification VV-G-1690C for leaded and unleaded gasoline. Detergency testing is also described using Federal Test Method Standard Mo. 791B, Method 550-“Induction System Deposit Tendencies of Motor Gasoline”.This paper concludes that careful selection of various fuel additive components can result in fuels that improve engine efficiency.