The standard method for evaluating the quality of automotive fuels is based on the determination of its motor octane number (MON) using the ASTM motor testing. The resulting figures of merit of this standard testing are known to be strongly linearly correlated to the fuels' vapor thermodynamic properties such as, for instance, their vapor pressure. In this paper we explore this strong correlation between the MON parameter and the fuels' vapor thermal properties to seek the development of an alternative low-cost method. The proposed method is based upon the photothermal measurement of the thermal diffusivity of the fuels' vapor in a closed air cell. It is found that the thermal diffusivity is well correlated with the corresponding MON values of the fuels investigated. The eventual improvements of the proposed method aimed at gaining accuracy and developing a portable device for field applications are also discussed.