Current political efforts to meet global climate goals require efficient propulsion systems for industrial and mobile applications. Optimizing efficiency also offers significant potential for energy cost savings. Reducing friction through tribologically optimized contacts helps minimize power losses and maximize load capacity. Gear friction is a transient phenomenon since tribological conditions change over the path of contact. Various models exist for calculating gear friction, but they show significant differences under the same conditions, particularly in the high-speed regime where experiments are lacking. To close this gap experiments were conducted to measure gear power losses using calorimetric properties in a high-speed back-to-back gear test rig. Factors such as pitch line velocity, surface topography, lubricant temperature and gear geometry were studied. Based on experiments, an existing friction model was calibrated adjusting parameters like topography, load, and speed. The pitch line velocity significantly influenced gear friction, with higher velocities reducing friction due to hydrodynamic effects. Test specimens with isotropic superfinishing showed a mean friction coefficient about 30% lower than ground gears. The adjusted friction model evaluates load dependent power losses, improves efficiency in mobility and aviation, contributes to more sustainable transportation.