No matter who wears the pants, trucks should be fitted for skirts. A recent public-private partnership in the Netherlands yielded that "side skirts" for the trailers of freight trucks reduce aerodynamic drag.

Side skirts are panels that attach to the sides of the trailer and extend the length of the trailer closer to the ground. Side skirts reduce air friction by minimizing currents under the vehicle. Reducing friction in this way appears to cut air resistance by nearly 20% and fuel consumption by 10 to 15%, subsequently reducing emissions.

Adding other parts that "close the gaps" and streamline trailers are similarly quick and relatively inexpensive means to trucking efficiency.  Many of these parts are already typically available, such as roof fairings (the curved hood above the cab), a cab extender (flaps that help the sides of the truck meet the trailer), and a front air dam (extension of the bumper from the grill), leading to modest efficiency gains of up to 5%.

Additionally, truck manufacturers often offer aerodynamic models whose more expensive initial cost will soon be quickly balanced by the increased cost in gas of driving inefficient models. In 2007 alone, SC Johnson Co. saved a reported $1.6 million through efficiency measures.

Read more at:
Sciencedaily.com
Green Car Congress