AbstractA method is presented for determining the efficiency of gear units in test rigs with a mechanically closed energy flow, based on measuring the losses in the rig, that is, the torsion torque at the driving motor and the stabilised thermic level. The energy losses between the two gear units would be separated according to the direction of power flow within the stressed framework of the rig.Experimental results are given for gear units including three types of gears, i.e.: involute gears with hobbed toothing, involute gears with ground toothing, and Wildhaber/Novikov (W/N) non‐involute circular arc gears with two meshing lines, achieved by hobbing.