What's the absolute max useable fuel in a Duster?

Yup. This here ^^^^ is the only way to know for sure.
Rusty - I'm a bit disappointed that I can only get 15.1 gallons of gas into my new 16 gallon tank and filler neck. I'm pretty certain that the tank was completely empty when I filled it. I removed the filler cap and sloshed the back end of the car back and forth with my ear to the open filler neck. I couldn't hear any fuel sloshing around in it. It seemed completely dry.