A new methodology to predict the transient operational temperature of a polymer–steel gear pair under loaded running is presented. For the involute gear form, rolling and sliding leads to a loss of gear efficiency and generation of heat in the contact zone. The power dissipated is used to set the conditions for a series of rod on disc experiments. The rod-on-disc data are processed in a time averaging procedure, which allows prediction of the complete gear temperature. This is assessed with analytical and finite element models to validate the predicted temperature rise against the experimental data. The significance is that the experimental procedures may be used to assess gear thermal performance without testing full gear pairs. •To predict transient temperatures in polymer–steel gears due to variable roll-slip.•Methodology to measure transient temperatures in polymer–steel gears.•Experiments to measure transient temperatures from steel rod on polymer disc.•Methodology to correlate measured temperatures with predicted temperatures.