Most likely it's one or BOTH fuel level sender units in the fuel tank (there are two float units, one with the fuel pump). Located under the rear pass seats on the driver's side and another on passenger side (seats need to be moved out of the way to get to them - four 14mm bolts with each seat).
You'll see the 8mm screws holding the units in place.
In this diagram, the sender units are wired in series (one or both may be causing your problem). The sender units are simple potentiometers (rheostats) & their combined resistance tell the microcomputer in the instrument cluster what the fuel level is, then It tells the fuel gauge where to put the needle. Inspect your wiring connectors to these sender units first. These sender units can be ordered/purchased at a Hyudai Parts Department. I don't believe there are any after-market units available - they are a dealer item.