The European Commission in joint collaboration with Heavy Duty Vehicle manufactures, the Graz University of Technology and other consulting and research bodies has been preparing a new legislative framework for monitoring and reporting CO2 emissions from Heavy Duty Vehicles (HDVs) in Europe. In contrast to passenger cars and light commercial vehicles, for which monitoring is performed through chassis dyno measurements, and considering the diversity and particular characteristics of the HDV market, it was decided that the core of the proposed methodology should be based on a combination of component testing and vehicle simulation. Emphasis is put on accurately simulating the performance of different vehicle components and achieving realistic fuel consumption results. A proof of concept was launched aiming to test and prove that these targets are achievable. A series of experiments were conducted on 2 different trucks, a Daimler 40ton Euro VI, long haul delivery truck with semi-trailer and a DAF 18 ton Euro V rigid truck. Measurements were performed at the Joint Research Centre’s HDV chassis dyno labs and on the road. A vehicle simulator (Vehicle Energy Consumption Calculation Tool - VECTO) has been developed to be used for official monitoring purposes and the results of the measurements were used for its validation. As inputs the simulation based methodology considers test track measurement of driving resistances (eg air drag), determination of drivetrain losses (e.g. gearbox), determination of power demand of engine auxiliaries (eg. cooling fan) and other consumers (e.g. steering pump), measurement of the engine fuel consumption map as extension to the engine's type approval tests (as described in EURO VI legislation). CO2 emissions of the vehicle are then calculated using the aforementioned input data for predefined representative driving cycles and mission profiles. For the two Heavy Duty vehicles tested and simulated on the same test route, fuel consumption was calculated always within a ±3% range from the real world measurement, and in several cases even closer than that (in the order of ±1.5%). Given the variability of the actual measurement (σ = 2%), it is concluded that a future certification scheme can be based on vehicle simulation tools. ; JRC.F.8-Sustainable Transport