My company uses side skirts on their straight frame trailers. I pulled the same trailer loaded (17k-25klbs cargo weight) and empty. Loaded I managed 9.3 from NE Tennessee up I-81-I-78-I-95 (NJTPK) - I-278 (Tolls into Brooklyn, NY) - I-495 to mid-Long Island, NY. I'm not sure of the empty MPG, but from Long Island through all the start and stops for tolls and traffic, I still manage 10+ from Long Island to Cleavland, OH. The next time I pulled a trailer without skirts in moderate wind, I stuggled to get 8.
edit:
Our trucks have all this, plus a more aerodynamic tractor.
http://bioage.typepad.com/.shared/im...ed/walmart.png