Fuel Gauge not reading accurately

I just recently purchased a 2004 Santa Fe GLS.  We filled it up right after we bought it and the gauge read full.  The next day we started it and it went to E and the fuel light came on.  Later on that day it read 1/4 tank.  Lately it has been reading 3/4 which could be accurate because we have not put that many miles on the new tank of gas yet.  Any ideas on what could be causing this problem?

It could be a bad float in the tank or a bad ground on the wiring.

I agree with NovaResource that this could be a bad ground wire connection, especially since the gauge jumps up and down between fills.  Old style floats with leaks would sink and then drain as the gas was used up to refloat briefly when the tank was filled, but that's not the symtoms you see.  Most newer floats are made out of a closed cell foam that I've never seen develop a problem (but all things are possible I suppose).  I'd start by check the ground wiring to the sending unit.

I had the same problem with 07 Santa Fe, and they replaced the Sending unit.....
 
The problem is starting again slowly when I fill up it is not going up again all the way to the Full level.... Another trip to the Dealer...
 
Hope this time will be good.
 
 

the sending unit on the pass side of the tank is not working properly. the car has two of them.

Yeah they have two and changed both of them at that time...  I will have to bing it back...