The theoretical model estimates the characteristic parameters of a car spark ignition engine by using complex functions. Analyzing the variation of the efficiencies and the fuel consumption at various operating regimes it is possible to optimize them according to the operating regime of the car. It is also studied a method for estimation of the specific fuel consumption of the spark ignition engine by considering engine operation in variable mode. The theoretical study presented by authors contributes actively to the optimization of the interaction between vehicle and propulsion system by identifying low fuel consumption modes.