•Developed thermo-mechanical model produces results consistent with exp. measurements.•Model enables the consideration of any type of spur gear tooth profile.•High-speed thermography offers detailed insight into thermal response of polymer gears.•Tooth flash temperature gradient quickly dissipates after meshing cycle completion.•Geometric tolerance deviations influence the temperature rise on a given tooth pair. Display omitted The presented work is focused on the development of a comprehensive thermo-mechanical model for predicting the temperature rise in thermoplastic polymer spur gears with any desired profile geometry while running. The specific constitutional behavior of thermoplastics influences the gear-meshing pattern, which can deviate substantially from ideal gear meshing, as typically exhibited by metal gears in moderate-loading conditions. Taking this aspect into account is of paramount importance if realistic temperature-rise predictions are to be made. The thermal response of the considered gear pair is studied thoroughly from both the analytical and experimental standpoints. Good agreement was found between the results of the model and the experimental measurements performed using a high-speed thermal imaging infrared camera, although it was also observed that the real-life temperature rise can increase noticeably if large geometric tolerance deviations from the ideal profile geometry are present. The presented experimental approach also offers the possibility to observe the temperature rise inside and outside the meshing cycle.