http://www.sti.nasa.gov/tto/Spinoff2008/t_3.html
Quote:
For a large truck, this can mean a particularly large surface moving a large quantity of air at a high velocity—its blunt face acting like a fast-moving bulldozer, creating a zone of high pressure. The displaced air must go somewhere, spilling around the cab into swirling vortices. The air traveling along the side moves unevenly, adhering and breaking away, and sometimes dissipating into the surrounding air.
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Rounding the vertical corners on the front and rear of the cab reduced drag by 40 percent while decreasing internal volume by only 1.3 percent. Likewise, rounding the vertical and horizontal corners cut drag by 54 percent, with a 3-percent loss of internal volume
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Since the rear of the truck is not flush with the front of the trailer, with even more of a gap due to the small cross section of the bed, the same aerodynamic principle applies to the front end of a trailer.