Gearbox efficiency is a topic of rising interest amongst both OEM and end-users due to an increased sensitivity to gearbox performance, reliability, total cost of ownership (energy cost), overall impact on the environment, and also anticipating future regulations. In a gearbox there are difference sources of losses: gear, lubrication, seal and bearing loss. The use of modern simulation tools makes easier the evaluation of losses in various load case conditions. It has been demonstrated that the contribution of bearing loss on the system efficiency is dependent on the load cases. Even if the bearing is by far not the primary source of losses, the optimization of the bearing set can significantly improve gearbox performance. Simulation of a single stage gearbox using tapered roller bearings shows that the running temperature of the gearbox can be reduced up to 10C, by using latest bearing generation. Such a saving could improve the thermal rating of the gearbox by up to 30%. Experiments also demonstrated that different design of tapered roller bearing shows significant variation in friction performance. Having proper bearing design can significantly improve the performance of a gear unit: by a lower running temperature, by improving lubricant life, potentially simplified lubrication system, and consequently reduced running cost.