just dropped mine today
dash gauge read 1/2 and i knew i had less , so I pumped the tank almost dry using fuel prime wire and a hose on the fuel rail with the shrader valve removed to point where bubbles , only pumped out 4 gallons so it was lower than 1/2 tank it showed on gauge
to test tank ...disconnect tank harness at tank , on harness to tank with ohm meter.. one lead to center pin (pink) and other lead to ground pin (blk wire)
on analog cars sender should read 0ohm with arm at bottom (empty) and 90ohm with arm at top (full)
if sender had showed 10ohm (and tank is near empty ) and my dash showed 1/2 tank then the wiring to the gauge or the gauge would be next things to check not the tank .
mine tested at 50 ohms ,
obviously the problem is in tank since dash gauge reads close to what sender is showing
so I dropped tank (really easy with a lift guys )
pulled the sending unit and thought i heard the arm drop
tested the sender on the bench and it was perfect (well good enough 4ohm at bottom 100ohm at top)
reached in and found the plastic bucket (stock tank fuel reservoir) moving around a little (about an inch) and apparently jammed the sender arm, it was still fastened at the fill side of tank but moved a little at the forward side , i pushed back toward passenger side away from where the arm sits ..
thought for a moment about how i could make it stay put , in wiggling it around i felt the tab hitting the top of tank at one of the raised areas , took a hammer and dimpled in the corner of the raised humps , that fixed it .
sometimes the sender needs cleaning , sometimes the sender is broken inside , either is easy to fix if you like tinkering , i fixed one with parts from a NA hanger i had in the trash pile