fuel gage, calibration?

I replaced the fuel sending unit on my 72 Dart because the gage didn t move. After replacing the unit, the gage seemed to be fine. However, it reads empty when in reality, I ve only used 4 or 5 gallons of gas. I bought the unit from an online vendor. It is the identical brand sold at local parts stores. What do you think? Is there a problem with gage? Fuel sending unit? Can the gage be calibrated? One thing I noticed is that the sending unit float rod was a little shorter. Maybe that s the problem? Thanks for any help. . .