Legal constraints concerning CO2 emissions have made the improvement of light duty vehicle efficiency mandatory. In result, vehicle powertrain and its development have become increasingly complex, requiring the ability to assess rapidly the effect of several technological solutions, such as hybridization or internal combustion engine (or ICE) downsizing, on vehicle CO2 emissions. In this respect, simulation is nowadays a common way to estimate a vehicle's fuel consumption on a given driving cycle. This estimation can be done with the knowledge of vehicle main characteristics, its transmission ratio and efficiency and its internal combustion engine fuel consumption map. While vehicle and transmission parameters are relatively easy to know, the ICE consumption map has to be obtained through either test bench measurements or computation. Experimental measurement gives the most precise data but proves to be a too much time consuming and costly procedure for decision support concerning ICE design. In this respect, numerical tools are more adapted, especially when multiple engine architectures need to be assessed. This paper presents a numerical tool for the generation of an ICE fuel consumption map, and illustrates its application by studying the influence of engine downsizing on the fuel consumption of a given hybrid vehicle.