It depends. My bone dry gas tank holds 15.3 gallons of gas. It has an 18 gallon displacment, but that doesn't mean that it holds 18 gallons. If it were my tank, it would have had 1/10th of a gallon.

They were designed with a certain amount of tilt, with the 9 having the largest air bubble. The volume is verified by other by other 9 owners at around 1/10th + -.

The tilt in the install was intentional: NHRA required vehiles to have at least an 18 gallon tank, and it had to be full when drag raced. The semantics comes into play on the definition of full.

Full was defined as up to the fill neck and made no mention of volume, meaning it was legal to have a 5 gallon buble of air in the tank when leaving the line.

Also, it will suck the scum off the bottom even when it's full. My quiestion is why would there be scum in bottom of the tank? It would have to be a real [censored] owner to have such a condition.