This short and to the point wiki article from Wired.com [2] will give you some do-it-yourself tips on how to find out if you are getting the best miles-per-gallon from your car; from pencil and paper to apps. Useful, even if unfortunately you are not going on vacation!
Links:
[1] http://www.library.northwestern.edu/libraries-collections/evanston-campus/transportation-library
[2] http://Wired.com
[3] http://www.library.northwestern.edu/related-pages/car-trip