A new model of gaseous fuel-air mixing that is based on the ideal gas law and the equation of continuity is applied to propane-air mixtures. The local degree of mixing and the rate of mixing are calculated using the mass fraction of fuel measured within an infinitesimal fluid element and the time rate of this mass fraction, respectively. According to the model, mixing is promoted by pressure, temperature and velocity gradients. High initial pressure reduces mixing caused by pressure gradients. Results presented here provide the isolated effects of pressure and velocity gradients on mixing. These results facilitate the development of high intensity and high efficiency combustors with special focus on reducing pollutants emission.