Dropping from 70 to 65 mph will result in a noticeable difference in MPG. Missing a fuel stop is gravy.
EG. see
http://ecomodder.com/forum/showthrea...you-15182.html
I also agree with mcrews about instrumentation: it's worth it to get a MPG display. A ScanGauge or equivalent is also useful since it reports coolant temperature (and saves your "max" value) - perfect for developing a partial grille block.