1976 Scamp fuel gauge problems

Okay, after getting all the mechanical tuned up and this car is running better than ever, but this is still a problem. I have a new sender in the tank and the resistance goes up and down very smooth and is spot on in value with what is in the tank, but the gauge does not. The first time I filled it, it went to full and after driving it for a while I noticed it was almost on empty, so I went to fill it and it took all of three gallons to get it back to full. I have isolated the fuel gauge from the old circuit board and I put a 6v regulator on it thinking the little extra power might help. Now it reads empty after burning four gallons and the resistance read off of the line at the cluster is right for that amount. Could the gauge be that far out of calibration or could it just be worn out and I need to have it rebuilt or something?