Gears with asymmetric teeth have unique potential for application in gearboxes, particularly when uni-directional loading is applied. Most recently, gears with asymmetric teeth have received much attention for use in applications that require high performance due to increased load capacity. Such applications include aircraft and wind turbine. These gears offer flexibility to designers due to their non-standard design. In asymmetric teeth, the geometry of the drive side is not to be symmetric to the coast side. In other words, the pressure angle on the drive side is greater or smaller than that on the coast side. Asymmetry between tooth sides provide vital in obtaining key properties, such as high load carrying capacity, low weight, low wear or low vibration. In order to effectively design asymmetric teeth, it is necessary to perform analyses on the efficiency of these gears under various loading. In this study, the results obtained on high contact ratio involute spur gears with asymmetric teeth are presented and discussed. The impact of a few design parameters, such as pressure angle or tooth height, on sliding velocities and friction is investigated and illustrated with numerical examples.