To get the best mileage your trucks capable of, you have to update a few sensors and clean up a few things (internally). If your not getting at-least 14 mpg, then I would say you can do better.. It's work tho

 

the work does not bother me its the $ sign.

 

Have you checked your fuel pressure lately by chance ?

This site provides so much info on this subject - searching is part of the work.

Here's a few things -
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Update -

O2 sensors (just your 2 forwards B1S1 & B2S1 ) - get them here - http://www.autopartswarehouse.com/se...20Ford%20F-150

Injectors -

http://cgi.ebay.com/ebaymotors/Fuel-...mZ280267979114

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Clean-

TB/Plenum, MAF - New GOOD synthetic oil in all drivetrain - cuts out the friction.


A fresh set of coil pacs doesn't hurt as well

 

What weight oil are you using? At this point, you may not have anything to lose going to a heavier oil or even an additive to get the rings sealing up a bit better. With that many miles and low compression, 5W20 or 5W30 may not be ideal any more.

Good chance just replacing the front O2 sensors and cleaning out the throttle body will help mileage considerably. A can of BG44K in the gas will clean marginal injectors.