An earlier version of 4 Green Stars story included an incorrect sales number for Toyota's Prius. The number has been corrected.DETROIT ( TheStreet) -- Green cars took center stage at the press preview of the Detroit Auto Show this week. In fact, two of the Big Three U.S. automakers, Ford ( F) and Toyota ( TM), focused primarily on expanding electric-car families. General Motors ( GM) had already rolled out the Volt with a massive publicity effort that included a national tour. At the show, GM CEO Dan Akerson told reporters: "The Volt will help us change how people think about Chevrolet."