U.S. automakers have tried but been unsuccessful over the last 30 years in developing new powertrain technologies that significantly improve miles per gallon (MPG) or reduce CO2 emissions. These unsuccessful technologies include fuel cell vehicles (FCVs), diesel hybrid technology (Partnership for a New Generation of Vehicles), Battery Electric Vehicles (BEVs), and Plug-in Hybrid Electric Vehicles (PHEVs). The only new technology that has improved MPG (and reduced CO2 emissions) significantly has been the Hybrid Electric Vehicle (HEV), first developed in Japan in the late 1990s by Toyota (Prius) and Honda (Insight). The most successful HEV has been the Toyota Prius; 50% of hybrid shipments have been this single model. Most of the 38 different HEVs available or discontinued offer only marginally better MPG compared to their equivalent conventional models with the exception of Toyota and Honda. The fuel economy improvement for the best HEVs is in the range of 30-40%.