Hi Steve,
I recently had to recalibrate my guage, so it may be one of two things:
1) the tank sending unit may need tweaking. It is a rheostat with a float, and it may be off a little. You can bend the float arm slightly to get it centered where it needs to be. I was able to move the float by hand from the top of the tank to see if it was max/min at the end of travel by pushing on the wiper. Be very careful not to bend the wiper that rests on the resistor, it is fragile.
2) the guage may need calibrated to suit the tank sender. This is difficult and not for the squemish. Look in the archives for more details, there is one thread which will refer you to a MG site where a talented fellow took his guage fully apart and explains the inner workings and how he calibrated it. I basically emulated two resistances, full and empty, on the bench and finally got the guage to work properly though it doesn't seem too linear. I also broke one of the hair size conductors and had to re-solder it, that was fun.....8^).
There is also a thread about having a sender in the tank reversed but can't recall the symptoms.
I hope this helps,