First off, do an actual check of the mileage.
Fill the tank. Drive. Hit a nearby interstate and verify that the odometer is reading true. You can also use the mile markers and a watch to verify the speedometer is close--60mph is 1 mile per minute. Refill the tank at the same pump with the truck facing the same direction. mpg= the miles you drove ( corrected by what you found out when you drove on the interstate), divided by the gallons used on the second fill-up.

Using the fuel gauge, especially on a Ford, to estimate how much gas you used is bad data. On most Ford trucks I have had, the gauge drops like a rock to 1/4 tank, and then hangs there a while.

If its really using that much fuel, the "check engine light" (CEL) should be on because the computer should know something is wrong. By the way, does the CEL come on for a few seconds when you start it up, and then do out? If not, look for a bad bulb, or worse, a truck that has been hacked up.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags