One thing you could try is to run it down to 1/2 tank and refill it. Then multiply by 2 to get total capacity.
1996 F150 Scab longbed, 5.8L, 4x4, 31x10.5 tires, 212k miles and counting. step bars, low-profile tool box, Euro-clear turn signals & headlights, Euro taillights, vent visors, class III Reese hitch, Tekonsha brake controller.
"when you have a hammer in your hand, everything looks like a nail."