The use of bio-fuels, like bio-ethanol mixed with gasoline, is growing interest in the automotive industry, to reduce the fossil fuels dependence. However, the air/fuel ratio for any blend is not found in the literature. It is found only for specific values such as: E10, E50 and E85 for example. For this reason, in the present work a mathematical model is presented to detertuine the stoichiometric air/fuel ratio of the ethanol-gasoline blend in the range between 0% to 100% of ethanol molar percentage in the blend. The model is based on combustion chemical analysis for any composition ethanol-gasoline. The optimum compression ratio for maximum network is also obtained, considering isentropic processes during compression and expansion. The analysis of the Otto cycle with the air/fuel model and optimum compression ratio is developed. Results show that the stoichiometric air/fuel ratio of the blend ethanol-gasoline is not linear. The maximum difference between air/fuel ratios predicted and experimentally reported is 7% in the whole range of the blend. The analysis of the Otto cycle, using the equation derived shows that the power and the torque decrease when the ethanol mole fraction grows. The equations obtained in this work can be used to predict the performance of internal combustion engines using the ethanol-gasoline blend in the continuous range of ethanol molar percentage between 0% and 100%.