Plug-In Hybrid Electric Vehicles (PHEVs) have been promoted as a potential technology that can reduce vehicles fuel consumption, decreasing emissions related with the transport sector and dependence from oil importations. However emissions and cost impacts of PHEVs are strictly connected with the power mix generation used to produce electricity and when their batteries are recharged. Potential effects of a PHEV fleet on the power grid have been analyzed under different charging strategies, obtaining a quantitative estimation of the additional load affecting the transmission grid. The environmental impact of these vehicles is estimated for the State of Ohio as well as for other different countries showing the great importance that generation technologies have on PHEVs final environmental performances. Cost analysis shows how these vehicles can obtain great operation cost reduction due to the decrease of gasoline consumption, proposing this technology as a possible solution for decrease oil dependency of the transport sector.