In this paper, we consider terahertz sources based on Čerenkov difference frequency generation in a dual-wavelength mid-infrared quantum cascade laser and calculate analytically the optical external efficiency of the out-going terahertz wave. The mid-infrared pumps operate simultaneously and have a common upper level. For short cavity lengths and low terahertz losses in the substrate, we find that implementation of the Čerenkov emission scheme in terahertz difference frequency generation quantum cascade lasers improves the optical external efficiency dramatically. The maximum value of the latter is close to 19%, but is only attainable if the laser exhibits negligible mid-infrared transmission losses.