Thanks to Sheriff Buford T. Justice & Dyno Don Mallinson
For all V8 SHO Ford officially declares the gas tank capacity to be 16 US Gallons. At about 12 gallons down your low fuel light will come on and most folks tank up at 12-14 gallons.
The fuel pump is located in the gas tank and is submersed requiring the liquid to keep cool. An over heated fuel pump will fail leaving you found dead on the side of the road. Be that as it may, several folks have pumped 21 gallons into their 16 gallon tank (with certified gas pumps), often enough to to prove that Ford wants us to keep that pump submersed at the cost of lugging about a 1/2 tank at all times.
Under the best of conditions a V8SHO can obtain 25 mpg highway, with a 21 gallon tank a 500 mile range is do-able. Many folks have done 450 miles on a tank full.
As you may expect, it is not worth it to ruin a fuel pump to get 450-500 miles on a tank full. But you don't have to panic next time you low fuel light comes on.
Just a bit of advice from the mechanics I know, and the manufacturers.
Running the tank dry or nearly dry is not recommended. If there is junk in the bottom of the tank, it can temporarily clog up the intake tube. This also puts more of that junk into your fuel filter, shortening its life.
Running the tank low runs the risk of overheating the-tank fuel pump, which uses the fuel as a coolant.
Low fuel tanks create much more moisture through condensation than a full or mostly full tank.
Keeping the tank as full as possible and once in a while using a quality fuel system water inhibiter/cleaner is the best routine.
I have used this "full tank" philosophy for many years in my old cars that get used maybe 100 miles a year. I have never had bad gas ruin a carb (that is a common diagnosis by poor mechanics to get lots of carb rebuilds and $, and is a favorite routine of power equipment shops that are not ethical), and by keeping the tank full I have never had a tank rust and the sludge at the bottom is non-existent. The inside of my carbs are always perfectly clean.
Replace your in-line filter from time to time, use quality gas from a busy station.
Besides, running a tank low runs the risk of you getting stranded sometime. I usually only run a tank to about ½ around town, and if on trips I try to fill before the ¼ mark to have lots of safety room. Besides, regular stops on long trips keep me alert.