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

Here's the latest: Today I ran the engine until it quit due to fuel starvation. When it quit, the fuel gauge was showing 3/8 full. I re-calibrated the empty tank to read "E" on my gauge by recalibrating my Meter Match unit. I was then able to fill the tank completely with 15.1 gallons of gas. This tells me that my brand new 16 gallon fuel tank doesn't really hold 16 gallons. There must be an unfillable air pocket at the top of the tank. Also, the filler neck fills up as well. So including the filler neck, my fuel system holds a maximum of 15.1 gallons. The engine quit after running 227 miles. Not very good range.