1996 gas gauge not accurate

I have a 1996 buick Skylark with 3.1 engine. I just had the fuel pump replaced in the gas tank. Now its not reading proper. The needle is on E but I still have at least 6 gallons left in the tank. The guy I had do it says that this good that it errors to the low side. I want it to read what is actually in the tank and not speculate that I have 5 gallons or 8 gallons left. What needs to be done to get it right? Also the needle moves very very slow when it does have a full tank.

did he replace the sending unit if not you might check the conection for the sending unit and fuel pump harness it sound like a ground problem try running a ground wire and touch the tank see if that help i have seen it before

He replaced the module and pump. I tried over the weekend placing a wire on the gas tank straight to the battery ground. Made no difference. I even scratched the tank to get a clean spot to attach the wire.