Peterbilt and Cummins have successfully demonstrated a tractor-trailer that gets just under 10 mpg under real-world conditions.

A single-digit fuel economy rating doesn't sound like much to brag about, but in the world of tractor-trailers, it's a massive improvement over the status quo. In fact, it's a full 54 percent increase in fuel economy over the average tractor-trailer on the road today. The corresponding fuel savings could save owners around $25,000 in diesel costs a year, and would reduce annual greenhouse emissions by 35 percent per truck.

The tractor-trailer was built as part of the Department of Energy's Super Truck program, in which five manufacturers compete to improve truck freight efficiency – a measure of how much fuel is required to move a particular amount of cargo – by over 50 percent. Of those efficiency gains, 40 percent can come from engine improvements, and 60 percent must come from weight reduction, aerodynamic improvements and eliminating parasitic losses in the drivetrain.

Most of the competitors have worked on waste-heat recovery and aerodynamic improvements. Some have examined hybrid drivetrains, and others have focused solely on tweaking existing technologies and reducing weight.

In the case of Peterbilt and Cummins, their Super Truck converts waste heat into power that's delivered at the crankshaft. The engine has been designed to reduce friction, while the tractor and trailer combination have been designed to reduce drag, and the truck contains lighter-weight materials throughout. The addition of an advanced transmission allows the engine to run at lower speeds.

The demonstrator averaged 9.9 mpg over 11 312-mile test runs from Fort Worth and Vernon, Texas. That's admirable, but the actual truckers posting over at Truckers Report wonder how it would handle on hilly routes or stuck in traffic.

Even if the fuel economy gains aren't as high in real-world use as in test runs, the technology on board proves that its possible to squeeze significant mileage out of existing technologies. Depending on how much a production model costs — this is a prototype, remember — it may save owners some money in the long run, reduce the cost of moving freight and that could lead to less expensive products for consumers.