with some yarn,
a hot glue gun,
and a 'chase car' with movie camera
I would bet that
some simple aero 'tweaks' could get the
Highway MPG up at little cost:

http://autospeed.com/cms/A_108656/article.html

the above is the first of a 5 part series from the 'Land of Oz'

This archived website allows you to find out your aero and tire rolling resistance with coast down test on flat pavement:

http://web.archive.org/web/2004080307322...tDownCalcs.html

But I personally like to coast down a 4 to 6 percent grade hill, then use the 'top out' speed on the hill and the equations in this article to find the Cd and rolling resistance:

http://www.etrucker.com/content/downloads/ccj0302.pdf