This article describes an investigation that has been conducted to assess the effects of the contact pattern design on the mechanical and thermal behaviors of plastic-steel helical gear drives. The maximum contact pressure, frictional power loss, transmission error function and operating temperature are determined for different designs of the pinion tooth surfaces. Several numerical examples based on loaded tooth contact analyses and steady-state heat transfer analyses are carried out considering different types of micro-geometry modifications of the involute tooth surfaces. The obtained results show that an appropriate design of the pinion tooth surfaces can help reducing the maximum contact pressure and the maximum operating temperature, increasing the durability of the transmission. The frictional power loss and the peak-to-peak transmission error can be diminished, improving the performance of the transmission and increasing its efficiency. Also, it has been shown that the design of the pinion tooth surfaces can provide some capability to the transmission to absorb angular misalignments.