http://www.landlinemag.com/Archives/...tmoremiles.htm

sample quote:

"Slice through the air
Aerodynamics play a big part in fuel economy. A truck’s coefficient of drag is the sum of many factors. One rule-of-thumb often mentioned at TMC is that all things being equal – of course they never are – a fully aerodynamic truck will do roughly 1 mpg better than a classic tractor.

TMC Recommended Practice RP 1111, Relationships Between Truck Components and Fuel Economy, indicates that shortening the gap between tractor and trailer to 25 inches can save from a half of a percent to 5 percent, depending on where it was set.

Air deflectors can save up to 6 percent, while a full roof fairing can save up to 15 percent. Other aerodynamic savings include: cab extenders, up to 2 percent; contoured air dam front bumpers, up to 3 percent; tractor side skirts, up to 3 percent.

RP 1111 also states that switching from lug-type drive tires to rib-type can improve mpg 2 percent to 4 percent. Worn tires, with less mass to flex, can be 5 percent to 10 percent more fuel efficient than new ones, so expect a drop when you replace tires. "
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Even Peterbilt
famous for its brick-like 'Macho' old fashioned rigs
is offering full aero truck designs
along with tweaks on the old body style

http://www.peterbilt.com/index_new_m...5&archivedate=

much more MPG improvement info