aerodynamic changes do help MPG at highway speeds

there are plenty more aero mods listed in the FAQ

at highway speeds your mirrors cost about 1 MPG
which is a bit surprising.

Have a look at the vortex generators on the top of the outside mirrors
on the new Lincoln MKX SUV, older BMW sedans, Mitsu Lancer rear roof
and the underside of the Mercedes ML500 outside mirrors

An airdam made deeper and closer to the ground with a strip of lawn edging
will improve highway MPG,
as will a suspension lowering...which does the same thing to the airdam

In one of the press releases for the Ram SRT10 a Chrysler vice president stated that the rear spoiler slightly reduced drag

check out the 'dimple patch' of rubber above the radiator grille of the new Toyota Tundra,
which appears to be a tricky aero mod to improve airflow over the windshield behind it.

The rear brakelight covers of both the Tundra and the new Silverado appear to be shaped to create a vortex (horizontal tornado) to lessen the low pressure area behind the truck tailgate....this is the same wind drag problem targeted previously by the GM and later Dodge 'tailgate thingies'

The 2002 Ram 'third gen' was a blunt design of Cd = 0.528
compared to the sleeker 1994-2001 'second gen' Ram that was modelled after the Kenworth T600 large 18 wheel truck and had a Cd= 0.42

The new Tundra claims a Cd = 0.37 for 2wd and 0.38 for 4wd

It is possible to measure the wind drag and tire drag of your truck
by yourself using this old JAVA application stored in the internet archive:

http://tinyurl.com/3aompw

but be aware that when it tells you a Cd multiplied by Frontal Area
the frontal area is in the metric 'square meters'
even if you used square feet in the boxes