The effect of a homemade complete hydroxyl system on automobiles’ performance under real operation conditions has been investigated on three gasoline-fueled four-stroke four-cylinder engine automobiles. All measured parameters were carried out by using the AVL DiTEST Abgas test and the Lancom 9 gas analyzer. The exhaust gases CO2, O2, CO, NOx, and CxHy, at the exhaust tail pipe and before the catalytic converter were measured. A significant reduction of 35–45% in the specific fuel consumption was observed. An in-cylinder chemical reaction model was developed to understand the impact of hydroxy gas on the combustion chemistry of engine. The in-cylinder pressure results showed about 3-bar increase in pressure peak at 1.2% H2 addition to the engine air/fuel mixture. Also, the mixture ignition temperature was reduced from 600 to 560 K. The exhaust gas species showed a 12% increase in H2O. Furthermore, the comparative t-test showed that the average CO volumetric concentration was reduced by 12%, while the average O2 was increased by 11%. On the other hand, hydrocarbon concentration was randomly varying with a maximum reduction of 35% at 2420 rpm. The origin of the observed significant influence of introducing H2 and O2 molecules on the combustion reaction mechanism and flame front stability has been explained.