Sounds like you have a bad sender or gauge,

 

I had a similar issue myself last week. I wasn't sure of the accuracy of the fuel guage and basically took a shot at making it home from the job on what I thought was still in the tank. Approx 10 miles from the house the truck starts to sputter and I thought "uh oh" not going to make it. Grabbed 4th gear and the engine caught again and just as I round the corner to get in to the gas station the truck flat quits. Made it to the pump, filled it up and low & behold only takes 11 gallons. I thought these tanks were larger. Now I drive it by the trip meter. Got 24.9 mpg on that tank of fuel. Took forever for the fuel to get pumped from the tank to the engine by the way. Need to look in to what's up with the fuel tank.

 

I know my 93 XLT holds roughly 19.9 Gallons (I have been on vapors, and put 19.6 gallons in at one time).

I am usually at about 18.5 Gallons when I fill up.