Wal-Mart’s fleet of about 7,200 semitractor-trailer trucks is already about 15 percent more fuel efficient than it was when the company announced its environmental goals in late 2005 (25 percent improvement in three years and a 100 percent increase by 2015), MSNBC reports. Cutting the average mpg by one gallon saves Wal-Mart between $35 million and $50 million a year.

The 15 percent gain has come mainly from a fuel additive mix, more fuel-efficient tires and Alternate Power Units that power different systems, like AC, when the truck is parked (according to research, long-haul trucks idle 500 to 3,500 hours a year, consuming up to a billion gallons of diesel fuel a year). Wal-Mart has information on some fleet changes it has made on its site.

New trucks that are already coming to market will yield about eight percent more efficiency due to more aerodynamic design and lighter components. Another few percentage points will come from tire innovations.

The 25 percent gain will apply to most of the fleet but not to all of it by late 2008. If other innovations are taken into account, the total effect could be a savings of 28 or 30 percent from the 2005 base by the time the entire fleet is updated in 2010. The biggest piece of the solution will come from future hybrid diesel-electric engines.