An experimental and analytical comparison of the efficiency of high-speed helical gear trains is presented. Analyses of the gearing losses were conducted. Test data from a helical gear train at varying speeds and loads (to 3730 kW (5,000 hp) and 15,000 rpm) was collected. A comparison of the results indicated that the operational conditions of the gearing system affects the loss contributions of the various mechanisms and therefore the overall efficiency of the gear system.