Fuel sending unit/gauge issue? Wrong sending unit?

So this has been a problem for a few years. But since the engine rebuild I've been driving the Duster a lot more. The fuel gauge reads the top 4 or 5 gallons in the tank then reads empty after that. I actually never knew it was doing it because the grounding strap was broken but i made a new one and found out about it. What is wrong? wrong sending unit?