Fuel Tank -16 gallons.....why can I only put in 10 gallons?

Prine said
Ok.....on my 1974 Dart Sport....Since I got the car...I have never been able to stuff more than about 9.5 gallons of fuel in the tank.

I figured it was a bad/worn sender...and ordered a new Pickup/Sending Unit....got the 3/8" pickup one....ran the fuel level down till the gauge showed empty and pulled the old sender and installed the new one.

Same Issue....its not a HUGE concern....but I sure would like to be able to fill up with more than 10 gallons for Highway Trips.

I cannot figure out WHY with a 16 gallon tank....I cannot fill it over 10 gallons.

I do not know of a smaller capacity tank...and I did not install this fuel tank....does such a thing exist?

Or what is the possible answers to this issue?

Was the first sender you took out an original 30 year old Sending Unit?

My repro sending unit does the same thing. Shows empty and I put like 10 gallons in it. Repro unit have a swinging arc mechanism that is totally different than OE.

Old sending unit I have put 15+ gallons in it. Actually when the original was dead on the "E" stripe it was out of gas I would be walking.