New Auto Meter Gauge/Sending issues

I have an auto meter gauge in my dart and a sending unit from Vans as well... my issue is a strange one as well.. If I fill the Dart up, it reads full... however, it will work its way down to reading 1/4 tank in no time... Found out the hard way that if it is reading 1/4 i still have a little better than half a tank of gas... trouble is once it gets down to a 1/4 it slows down and doesnt move for a good while.. So while I have taught myself that 1/4 means half or better... I have no earthly idea whats going on from there.. havent pushed the issue yet, but I get the impression I would run out of gas with the gauge still reading 1/4 lol