A model to predict friction-related mechanical efficiency losses of hypoid gear pairs is proposed@ which combines a commercial available finite element based gear contact analysis model and a friction coefficient model with a mechanical efficiency formulation. The contact analysis model is used to provide contact pressures and other contact parameters required by the friction coefficient model. The instantaneous friction coefficient is computed by using a validated new formula that is developed based on a thermal elastohydrodynamic lubrication (EHL) model considering non-Newtonian fluid. Computed friction coefficient distributions are then used to calculate the friction forces and the resultant instantaneous mechanical efficiency losses of the hypoid gear pair at a givenmesh angle. Themodel is applied to study the influence of speed@ load@ surface roughness@ and lubricant temperature as well as assembly errors on the mechanical efficiency of an example face--hobbed hypoid gear pair.