Fuel gage off with fuel cell

I have a 71 Dart with the factory gage. Currently running a 12 gallon Jaz cell with their 70-10 ohm sender, 70 empty and 10 full. If I fill the cell to within 1" of the top, the gage will indicate about 3/4 full. After using only about 4 gallons the gage is on E. It sounds like I have too much resistance at the gage or possibly not a good enough ground at the gage. Any other thoughts?

Thanks..........!!