<snip> i have driven 15 miles and haven't even seen my gauge move a bit.
Oh nooooo!!! now your guage is broken?
Just kidding...

) but changing your fuel filter will not double your gas milage.
Your calculations have to be off somewhere.
The most accurate way to measure gas milage is to fill her up, and record the miles on the odometer.
Drive her down to about a 1/4 tank, and fill her up again, and record the miles on the odometer. Subtract one from the other, and see how many miles you traveled. See how many gallons you burnt up (the number gallons it took to fill her back up), and divide the number of miles by the number of gallons to get miles per gallon.
IF after doing this you are still under 12mpg you might want to consider if your driving habits are ******* gas. If not, you should pull diagnostic codes to see if any of your sensors or actuators are not functioning properly.
Basically your idle, fuel injection, timing etc are all controlled by your computer. It uses sensors to find out what is going on, and actuators to make adjustments. Since thay are so interelated and interdependant, there are a number of sensors that could come into play, so rather than get a list of possiblities the best thing to do is to pull codes.
Here is a thread that I put together, that I request that you use as instructional for pulling codes.
It really is easy. WIth an '86, the only special tools you will need is a 4 inch piece of wire, and a cheap $8 analog voltmeter.
http://broncozone.com/forums/index.php?s=&...ost&p=74581
Then please let me know what if anything is not clear enough so that I can edit it so that everyone will understand.
Your feed back will be greatly appreciated.
Thanks
joe