For environmental and economical reasons, reduction of fuel consumption is becoming a major concern for car and truck manufacturers and customers. Analysis of consumer way of using their cars indicates, that it is by working on low temperature and low engine rotating speed that benefits given by the lubricant can be optimized. Analysis of customers way of using their trucks indicates that the engine is mainly working at stabilized temperature, in hydrodynamic conditions; in such conditions (for exemple highway), the reduction of fuel consumption coming from the engine lubricant is rather low, compared with passenger car applications.On that basis, specific rig tests have been developed to help the lubricant formulator. Each of these tests is specifically designed, for different engine friction area study: valve train, rings/liner, crankshaft bearings. The correlation between engine friction on each area and fuel consumption reduction is discussed. The influence of lubricant formulation on friction reduction is studied with these rig tests.For passenger car applications, the best compromise with the respect of API and ACEA specifications is obtained with a 5W-30 formulation, which gives a significant global engine friction reduction compared with current products. Evolution of fuel efficiency properties with lubricant ageing during drain intervals is discussed after field tests on engine test observation.For heavy duty truck application, the best way with the respect of API, ACEA and truck manufacturer specifications is obtained by a set of three lubricants (engine + gear box + rear axle), which gives an attractive reduction of fuel consumption.