In this study, the influence of basic design parameters and tooth surface modifications on the mechanical (friction induced) power losses of a helical gear pair is studied. A helical gear mechanical efficiency model based on elastohydrodynamic lubrication (EHL) is introduced. The model is used to simulate the gear contact conditions of an example helical gear pair within the ranges of basic design parameter such as pressure and helix angles, number of teeth (module), and major diameters to quantify their impact on mechanical power losses. Variation of gear efficiency with these parameters are then weighed against other functional requirements such as transmission error amplitudes, and contact and bending stresses to demonstrate that many designs that have high efficiency might perform poorly in terms of noise, pitting, and tooth breakage. A representative design that is acceptable in all aspects is considered next with varying amounts of tooth modifications to demonstrate their impact on power losses. At the end, recommendations are made on how to reduce helical gear mechanical power losses while meeting other functional requirements as well.