Last October, Wal-Mart announced a startlingly ambitious conservation effort–a plan to double the fuel efficiency of its entire 7,000-tractor trucking fleet, to an average of 13 miles per gallon, by 2015. That could yield annual savings of $310 million. CEO H. Lee Scott Jr. went on to say that Wal-Mart would “show preference” to suppliers that “aggressively” cut back on their own emissions. (Translation for suppliers: Do it.)

The mega-retailer sketched out the technologies it was applying in an experimental truck, but when we asked for details, we got this not-so-friendly reply: “We have shared all the information with you that we have available at this time.”

Well, then. We went to Ken Howden, director of the U.S. Department of Energy’s 21st Century Truck Partnership, a research effort with other agencies and 15 private companies. Howden is a man who knows his trucks. So we asked him: How much are Wal-Mart’s improvements really worth? Here’s his take.

Aerodynamic Package “Yeah, the rooftop spoiler is the classic example of aerodynamic packaging. But adjusting the entire shape requires an entire design cycle, and there are regulatory questions involved. For instance, you can close the gap between the tractor and the trailer; that might give you a 2% to 3% gain, but you’d have access and safety issues.” Auxiliary Power Unit “APUs are put on as an aftermarket device [for generating electricity in the cab]. They’re not as clean, efficient, or quiet. People haven’t adopted them because they add weight and hurt the aerodynamics. We’d like to come up with a market-based incentive for truckers not to idle. If it saves them money, they’re going to do it. That’s what we’ve found with a lot of these technologies: People will do it if the incentives are out there.” Trailer side skirts “Side skirts entrain the air more efficiently under the trailer and keep crosswinds from causing turbulence under it. They can be made out of all kinds of materials, including flexible broom bristle. The trade-off is that if they’re rigid, they may hit train tracks or obstacles; if they’re soft and flexible, they’re not as efficient. And maintenance issues are a concern with all of them.” Super Single Tires “One wider tire takes the place of the two tires. The perception by the trucking industry is that two tires are safer than one, but that may not be entirely true. Sometimes, with two tires, drivers don’t even realize they have a flat, and the other tire is holding the load. That can be a safety problem. The super single tire is a little more expensive than one regular tire, though it’s easier to remove for maintenance.” Tag Axles “This sort of simple axle would offer the advantage of low friction.” (A tag axle reduces weight by eliminating the internal axle drive chain.)

Howden’s estimates add up to a truck that’s between 11% and 13% more fuel-efficient. Not bad for a start. But, we asked, can Wal-Mart double its fleet’s efficiency by 2015, as planned?

[Long pause] “Uh, heh. We don’t want to say they can’t get there–we like setting aggressive goals like that,” Howden says. “We’re willing to work with them to get there. This kind of goal setting is important.”