ok first the guy with the o2 sensor comment thats funny! 84 S10 with o2 sensors, good laugh.
ok well i own an 86 C10 burban and i have found that
replacing the air filter helps a ton.
carb is well tuned, dont turn higher then 800 rpm at idle if you dont have 2,(might have to raise in winter if u live somewhere close.)
be smooth in your accelerating, allow early shifts.
keep your tires well inflated, it does help alot.
take out anything you dont need, less weight the better.
dont fill up all 30-45 gallons, fill half way, saves weight.
roll stop signs, there so useless(unless people r there)
dont run your ac if you can help it.
good alternator, belts dont slip, or screech.
good spark plugs, (ensures u use all the fuel)
thats all the cheap easy things other then that, if you went all out, swap carb for smaller ports, change the gears.
good luck bud, i know how ya feel though, i get 10 mpg with my 454
