How to test unknown fuel gauge.

Yeah I ditched that factory point-type power interrupter general averager, years ago. There was a magazine article that came out in 99 or 2000 or close to that, where a guy built one from a 5v computer part and a resistor IIRC. Well I couldn't get one in my dash fast enough. That solved the drifting inaccurate needle. I think that was an E-burg story; possibly as late as 2003.Probably Mopar Action: I was big into that rag.
Then I dropped the tank and bent the pick-up sock to sit on the bottom.
And finally figured out where the sender had to be to show E on the gauge. Then I poured one gallon of gas in the tank and bent the float arm to indicate E on that amount of gas. Badaboom, now I have exactly 1 gallon in there when it says E. And the new electronic-regulator holds the needle dead-nuts accurate.

OK that sounds too easy. Yeah it took several tries to get the float to the right height,lol.