Fuel Gauge Problem

I just installed a new fuel sending unit in my 72 Dart Swinger. Now that I have it on the road, the gauge seems to work, however, when the gauge reads empty, I can only put 6 to 7 gallons of gas in the tank. I have already checked the ground on the sending unit and it didn't make a difference. Just wanted to know if anyone else has seen this before I pull the sending unit to check for the correct resistance.