The gear efficiency is considered as an important factor to design a high-quality gear with longer life. Efficiency-enhanced asymmetric spur gear system ensures fuel economy, and the emission of pollutant gases to the environment is less due to less fuel consumption. The heat generation within the gear box can be reduced by improving the gear efficiency which will reduce the gear failures such as fatigue and scoring. The profile modification technique is considered as one of the effective methods to enhance the spur gear efficiency. In this work, the symmetric gear tooth profiles of the spur gear are modified into asymmetric gear by employing two different pressure angles at the pitch circle of drive and coast sides. An accurate distribution of correction factors to the asymmetric pinion and gear (x1 and x2) is necessary for designing the asymmetric gear drives with maximum efficiency. Hence, the main objective of this investigative research work was to estimate the optimum values of correction factor of the asymmetric pinion and gear for maximum gear efficiency. A combined model of finite element tooth contact analysis and Xu’s friction equation is used to evaluate the gear efficiency and optimum values of correction factor. In addition, the influence of gear ratio, teeth number in pinion and input speed is studied on optimum values of correction factor x1 and the respective maximum efficiency with different asymmetric spur gear drives (S−, So and S+). Using the obtained results, equation has been developed to calculate the addendum modification coefficient of pinion (x1) for maximum efficiency through curve fitting method.