A thermally coupled efficiency model for a simple dip-lubricated gearbox is presented. The model includes elastohydrodynamic friction losses in gear teeth contacts as well as bearing@ seal and churning losses. An iterative numerical scheme is used to fully account for the effects of contact temperature@ pressure and shear rates on EHL friction. The model is used to predict gearbox efficiency with selected transmission oils whose properties were first obtained experimentally through rolling-sliding tribometer tests under representative contact conditions. Although the gearbox was designed using standard methods against a fixed rating@ the model was used to study efficiency over a much wider range of conditions. Results are presented to illustrate the relative contribution of different sources of energy loss and the effect of lubricant properties on the overall gearbox efficiency under varying operating conditions.