Concerns regarding global warming have increased the pressure on automobile manufacturers to decrease emissions of CO2 from vehicles. Diesel vehicles have higher fuel economy and lower CO2 emissions than their gasoline counterparts. Increased penetration of diesel powered vehicles into the market is a possible transition strategy toward a more sustainable transportation system. To facilitate discussions regarding the relative merits of diesel vehicles it is important to have a clear understanding of their CO2 emission benefits. Based on European diesel and gasoline certification data, this report quantifies such CO2 reduction opportunities for cars and light duty trucks in today's vehicles and those in the year 2015. Overall, on a well-to-wheels per vehicle per mile basis, the CO2 reduction opportunity for today's vehicles is approximately 24-33%. We anticipate that the gap between diesel and gasoline well-to-wheel vehicle CO2 emissions will decrease to approximately 14-27% by the year 2015.