Fuel tank size?

[SultanGris]

How big is the fuel tank on these cars, my car runs like crap below half a tank seems like, I had 1\3 left and put in 11 gallons, they aren't over 20 gal are they? Thinking maybe jet pump is f'ed up.

 

[Pauly22]

They are 18 gallons

 

[SultanGris]

I guess that maths out pretty close then, maybe I'm wrong about the jet\transfer pump. Thanks