You really can't expect to get all the mileage mathematically possible out of the fuel tank; it has a sump which is below the fuel pickup. You can't get that gas into the engine, by design. It's there to allow crap in the fuel to settle out and NOT get into the engine.
Pretty much why, to this very day, I will never run any vehicle below 1/4 tank. "You're gonna suck all that crud up into your filters!"My butt always puckers if my fuel gets that low. My uncle has been a mechanic for 50+ years and all I ever heard growing up was "Don't trust those g## da## gauges" and "You have any idea how much sh## is in the bottom of the tank you are gonna start sucking up and :: insert 40 minute technical lecture on topics I have no idea about::
I actually had this happend frequently in my 40; I had to remember which way to park on a hill or I'd "run out of gas" when the fuel was low in the tank!Not to mention if you experience any significant inclines or off-camber situations, there may not be fuel present at the pick up when that low.