Typically, the sending unit in the tank is messed up. You
may end up having to buy a pump/sender unit. Knowing
the gas mileage of my 560,000 mile Ford Escort, I opted to
watch the number of miles I travel and fill up whenever I
reach that number. The pump/sender was more money
than I wanted to spend. I used to see this problem from
brand new trucks: they would be shipped by rail with 1/8
tank of gas in both tanks - the potentiometer part of the
sending units would rub a thin or bare spot from bouncing
in the same place all the way across country. The easiest
way to access the pump/sender? Pull the bed. 6 bolts, the
filler neck and the wiring. You don't even have to take it
completely off of the truck if you don't want to. Be sure
not to try to move it alone.