This paper reports the results of experimental tests performed at ENEA (Italian National Agency for New Technologies, Environment and Sustainable Development) in its “Casaccia” Energy Research Center to evaluate the energetic and environmental performances of a Heavy-Duty Compressed Natural Gas (HD CNG) engine fuelled with a hydrogen-methane blend of 15% in volume. A lean burn Mercedes 906 LAG engine has been optimized properly calibrating ECU engine maps regarding both ignition advance and air to fuel ratio (AFR). It was therefore possible to correct ignition advance to take into account the faster combustion speed given by the hydrogen content of the fuel mixture. Equivalence ratio (Lambda) has instead been modified in order to minimize the NOx emissions. All the tests were performed on a steady engine test-bed focusing the attention on the most important parts of the engine maps. The target of the activity was the reduction of energy consumption and CO2 emissions, still catching targets on pollutant emissions to getting the homologation of the engine in the same environmental category of the original one.Experimental results showed a reduction of CO2 variable with the ignition advance: decreasing the ignition advance timing the advantages are reduced. A reduction of CO2 of about 9% can be observed, for the same overall levels of pollutant emissions.After the calibration procedure the engine has been object of formal EU homologation and is running on a city bus since the start of 2013. Results about the on-road experimental campaign (which will last 150.000 km) will be published in future.