They could start by requiring pickups to meet the same fuel efficiency requirements as cars when purchased by noncommercial users. The idea that a GT500 comes with a gas guzzler tax while some soccermom can go buy an Expedition for the urban jungle without paying a tax is absurd. Level the playing field a bit and people who have no need whatsoever for a big SUV may think twice before buying one. Reduce the ratio of pickups to cars, and CAFE improves.
If the Corvette goes, we're all screwed. The regular C6 gets a lot better gas mileage than any Mustang.