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Air-fuel ratio control in gasoline engines has so far relied upon the fact that the stoichiometric air-fuel ratio of gasoline is an identified constant, largely thanks to its consistent chemical composition. In the case of Liquefied Petroleum Gas (LPG), chemical composition is subject to high variability due to geological and economic factors, amongst others. The implication of this variability is unpredictable stoichiometric air-fuel ratio of the fuel supply within any given vehicle, and ultimately degraded control of air-fuel ratio. This paper addresses the problem of stoichiometric air-fuel ratio estimation by evaluating the measurement and modeling of the relative permittivity of fuel, and also the method of iterative computation. For the estimation method proposed in this paper, simulation results are presented to demonstrate its effectiveness.