•Spur gear bulk temperature modeling.•Non-Newtonian thermal mixed EHL modeling of gear contacts.•Coupling of bulk and flash temperature formulation to find max. temperature.•Demonstration of roughness and speed influences on scuffing torque. This study proposes a scuffing model for spur gear contacts. A heat transfer formulation is devised to evaluate gear bulk temperature, with frictional heat flux yielded from thermal mixed elastohydrodynamic lubrication (EHL) analysis. In return, the bulk temperature is fed back into the EHL modeling to determine tribological behavior within contact zone, including flash temperature. After bulk temperature stabilizes, the bulk and flash components are added to arrive the total surface temperature, whose maximum is compared to a scuffing limit for failure determination. Utilizing the model, a set of simulations is performed to demonstrate the influences of surface roughness and rotational speed on scuffing load.