This article investigates theoretically the products from adiabatic combustion of synthesis gas reformed out of aqueous methanol with a variable gasoline-methanol mixture assuming chemical equilibrium. Previous research has generally focused on the combustion of a given fixed fuel composition, whereas in this article the composition of the fuel is varied by three different parameters: the amount of water in the aqueous methanol used as the synthesis gas feedstock, the amount of methanol in the gasoline-methanol mixture, and the ratio of synthesis gas energy to total fuel energy. The effects of these parameters on the adiabatic flame temperature and combustion product distribution are determined. The method used in this article allows the above parameters and the air/fuel ratio parameter to vary in equation sets derived using Lagrange undetermined multipliers in order to determine the combustion temperature and products with a computer solution. Stoichiometric combustion of synthesis gas reformed out of an equimolar water-methanol mixture causes a decrease in the NO ratio by 20% compared with methanol and 40% compared with gasoline under equilibrium conditions, whereas the adiabatic flame temperature decreases by only 50 and 100 K, respectively. Stoichiometric combustion of synthesis gas reformed out of neat methanol causes a higher adiabatic flame temperature and higher NOx emissions than stoichiometric combustion of gasoline or methanol. However, neat-methanol-based synthesis gas has a higher power density and extends the lean flammability limit of the engine more efficiently than aqueous-methanol-based synthesis gas because the reforming enthalpy of neat methanol is higher than that of aqueous methanol. Hydrogen-rich synthesis gas fuels should be combusted with lean air/fuel ratios at part load to increase the thermal efficiency, combustion stability, and control emissions simultaneously. If the road load increases, the engine charge's energy density and motor octane number will be increased by substitution of synthesis gas with liquid fuel while decreasing the air/fuel ratio simultaneously.