Couple of thoughts here.
Breathing is a factor
maybeif you keep the high # gears.
The easy way to get better mpg is to lower speed. Engine and vehicle.
If your engine speed (rpm) is down better breathing wont mean much as it breaths plenty at low rpm's.
Aero gains should be considered, like LHM said about holding your hand out the window at speed. Try holding a truck mirror vs. a sport mirror.
Bed cover, Grill block, narrow tires?, vac gauge.
Take a look here....
http://ecomodder.com/forum/fuel-econ...ifications.php
There are a few truck guys on that site. Some of there ideas work but I wouldn't do them. Engine kill switch, teeny tiny car.
Some good ideas.