An increasing interest in the use of natural gas in CI engines is currently taking place, due to several reasons: it is cheaper than conventional Diesel fuel, permits a significant reduction in the amount of emitted carbon dioxide and is intrinsically cleaner, being much less prone to soot formation. In this respect, the Dual Fuel (DF) concept has already proven to be a viable solution, industrially implemented for several applications in the high duty engines category. Despite this, some issues still require a technological solution, preventing the commercialization of DF engines in wider automotive fields: the release of high amounts of unburned fuel, the risk of engine knock, the possible thermal efficiency reduction are some factors regarding the fuel combustion aspect. DF configuration examined in the present paper corresponds to Port Fuel Injection of natural gas and direct injection of the Diesel Fuel.The paper reports a theoretical study on the adoption of natural gas for the specific application of the DF concept to Light Duty Diesel Engines for automotive applications. Several works explore the behavior of relatively small Diesel Engines in Dual Fuel mode, but it is more difficult to find, in literature, an evaluation of the natural gas potential in terms of carbon dioxide savings on light duty engines. The present study aims at performing this estimation developing a methodology for the Tank-to-Wheel and the Well-To-Wheel evaluation on transient driving conditions (e.g. the NEDC homologation procedure). In order to calculate the TTW factor for Diesel engines of current technology, fuel consumption (FC) data for a Euro 5 in a DF Diesel-NG version are necessary but not available in literature. Therefore, starting from FC data of an operating map for a Euro 3 class engine modified and optimized in DF NG-Diesel version, such data were properly updated for Euro 5 class engines. Then it has been possible to derive the global NEDC FC value by means of a consolidate procedure of FC estimation from steady-state data.Within the limit of the methodology assumptions, the results show a great potential of the CO2 reduction beyond a CNG substitution rate of 50-60%. With respect to the experimental data such limit appears achievable with a proper calibration of last-generation automotive diesel engines.