A study on the overall performance of an engine powered with hydrogen-enriched NG at stoichiometric condition, for different hydrogen shares have been described in this paper. The research has been carried on a General Motors Company X16SZR 4-cylinder, 4-stroke 1600 cm3 engine. Engine dynamometer tests were complemented with mathematical model calculations. Tested engine has been equipped with an aftermarket CNG feeding system where fuel is being injected into intake manifold simultaneously under low overpressure. Research program provided analysis for fuel blends with variable methane/hydrogen volume proportion (%): 100/0, 95/5, 90/10, 85/15, 80/20, 70/30, 60/40 and 50/50. Ignition timing and all other strategies, excluding EGR, remained unvaried. Testing procedure provided three different steady-state engine operation points for each of 8 different fuels: idle, high speed without load and full power at speeds in range of 1500-3500 rpm. The main aspect of the analysis was to identify the influence of hydrogen share on engine parameters such power, fuel consumption, in-cylinder pressure, temperature and exhaust gas composition. Very significant possibility of CO₂ emission reduction has been identified. Authors do also attempt to identify optimal blend for applications without any significant changes into engine construction and ignition/timing calibration. All possible negative effects of increasing hydrogen share have also been taken into consideration - like knocking. Analysis carried out on the basis of results allowed it to the point on methane/hydrogen proportion in fuel blend considering a total efficiency, emission and heat flux in parts of combustion chamber in engine. The results provide a basis for further studies on optimization of the process of managing the process of combustion in car engines fuelled by CNG and equipped with fuel reformer or using natural gas and hydrogen mixtures prepared at a filling station.