Back in the early 90's i picked up a 76 d100 with only 46k miles on it from the original owner. Truck was so original it still had it's born with plugs, wires, belts and hoses... Fuel guage never read below a 1/4 and would never read full...... I'd fill it up and go 250 miles and stop for gas...never ran out that way.
I realize this does not answer your question but, it was the nature of the beast.