Wal-Mart has improved the fuel efficiency of its 7,000-truck fleet by 20 percent since Oct. 2005 and is on track to meet its goal of a 25 percent improvement by the end of 2008, a Wal-Mart executive said Friday. Having already downsized its diesel tanks and started rolling on more efficient tires, the company also hopes to introduce hybrids into its fleet and perhaps devise a scheme to keep refrigeration running while the engine is off. “With gas at $4 a gallon, we’re keenly working on reducing costs,” says Matt Kistler, senior vice president of sustainability at the retail behemoth. Wal-Mart-owned Sam’s Club has also started carrying milk jugs made of 5 percent less plastic, and, in non-eco-news, Wal-Mart will be the sole vendor of the upcoming AC/DC album. Rawk!