A fundamental requirement for natural gas (NG) and renewable methane (e.g. bio-methane or power-to-gas methane) as automotive fuel is reliable knock resistance; to enable optimization of dedicated NG engines with high compression ratio and high turbocharger boost (which enables considerable engine downsizing factors).In order to describe the knock resistance of NG, the Methane Number (MN) has been introduced. The lowest MN which generally can be found in any NG is 65, and the vast majority of NG (~ 99.8%) is delivered with a MN above 70. The MN of bio-methane and power-to-gas methane is usually far above 80. Thus, from an automotive point of view any methane fuel should at least provide a minimum Methane Number of 70 at any point of sale. But the European draft standard describing the automotive CNG fuel quality so far proposes a minimum MN limit of 65. Therefore the efficiency difference caused by a MN reduction from 70 to 65 is of considerable interest for future NG (methane fuel) standardization.In order to investigate the MN effect on the efficiency and CO2 emissions of a dedicated future NG passenger car engine, a workhorse engine with increased combustion pressure capability and increased CR was designed and built. Steady state engine dynamometer testing was performed with different NG fuel qualities (different MN) within the framework of the European research project “GasOn” supported by the European Commission.The engine data and CAE (Computer Aided Engineering) analysis were used to determine the efficiency and CO2 impact of a Methane Number reduction. A reduction from MN 69 to 64 leads to an efficiency reduction of approximately 0.3 % for an engine with small cylinder bore (71.9mm). Larger efficiency reductions will occur for engines with larger cylinder bores, and for future engines with higher boost levels and/or higher compression ratios.