Abstract: An algorithm permitting the determination of the viscosity-dependent power loss in an automotive engine was generated. Using Tapered Bearing Simulator (TBS) Viscometer data at 100°C and 150°C from the Institute of Materials (IOM) Engine Oil Database, the new algorithm was applied to 1300 oils purchased in North America, Europe, and Asia in the years 1999 and 2000. When both the claimed "fuel efficient" oils and the oils not so marked were compared using the new algorithm, the results produced reasonable views of the fuel efficiency which could be expected from the oils of these three databases. The new algorithm has been termed the Fuel Efficiency Index. An Appendix compares the Fuel Efficiency Indexes of all the averaged individual North American SAE grades.