Power losses in high-speed gears come from the friction between the teeth (sliding and rolling), the lubrication process (dip or jet lubrication), the pumping of a gas-lubricant mixture during the meshing, and the losses associated with windage effects. The authors have developed different approaches to analyse the contribution of each power loss source, namely: (a) the windage losses based on simplified air flow models neglecting the influence of the lubricant, which compare well with the experimental evidence from a specific test bench; (b) an original model of gas trapping in the intertooth time-varying spaces, which has been validated using the experimental findings on a spur gear test rig in which pressure transducers have been placed at the bottom of the space between two teeth; (b) tooth friction, by introducing into a three-dimensional dynamic model of gears a new traction law based on measurements from a two-disc machine which accounts for lubricant properties and surface finish. The combination of the three models allows for the prediction of gear efficiency over a wide range of speeds and loads for jet-lubricated geared units. This approach also makes it possible to improve tooth geometry and gearbox design to minimize power losses and heat generation. The procedure is illustrated in several industrial applications.