True but whatever is at the outer edges when full will find its way to the pickup when run dry. I used to dyno test a lot of diesels several years ago and as fuel temperature went up horsepower would drop especially as fuel temp went over 100*. Engines would lose 10% and more as fuel temps went higher.

 

The ford fuel gauge is VERY conservative on the bottom end. Only being able to to put 12-15 gallons in a 19 gallon tank is normal. When I installed my gauge face overlay I "adjusted" the needle to read more accurate on the bottom end of the tank. That combined with the quick filler neck mod allows me to fill my truck with both tanks around 1/8 to empty with 32 gallons of fuel. Rather than the previous 25 gallons.

as far as running the tanks low (Not dry) I do it all the time. I would rather suck up a little junk over a lot of time rather than HAVE to runt he tank low before i can fill and suck it all up at once.