All aftermarket senders have the same issue, the sender is not calibrated for our cars gas tank shape and the curve the gauge requires.
THESE NUMBERS ARE FOR EXAMPLE ONLY I PULLED THEM OUT OF MY BUTT.
THEY REPRESENT THE SHAPE OF THE GAS TANK
So the first 5 gallons or so lower the tank level about 6 inches, the next 10 gallons, another 4 inches, and the last 3 gallons the final 2 inches.
So the liner sender shows full for several gallons, then get in the 3/4 to 1/4 of its sweep range and shows 1/2 to 1/4 for the rest of the time till you are at the last 3 gallons range and then shows empty.
Some will argue that's fine because you only need to know when the tank is empty.
My counter is I want to know when I have 18, 13.5, 9 and 4.5 gallons (I know they are not that accurate) but then I can judge about how far I can go with the fuel I have left.
There is a device called a meter match that has seen great success.
Or you can look for a good used sender, or get a used sender rebuilt.
Look at the chart.
Product Review: A100 Fuel Sender For 67 Dart