Gas Gauge

When I fill up my gas tank the gauge only shows a quarter of a tank. If I drive it for a couple of days untill the gas in the tank gets to a quarter or less the gauge works. It only works from 0 to a quarter. I have replaced the sending unit but its still the same. Note: When I take the ground wire off, which is located near the gass tank, the gauge will go to full and stay their.

85 Buick Regal

Thanks for your help.
Steve

I believe there is a resistor that you need to remove inside of the fuel tank, i'm not 100% sure, but i've had to deal with this kind of problem before. The resistor is what guages the needle. Unless it's a float, then something is wrong with it, i'm not sure exactly what device is used but i'm positive it's inside the fuel tank.