Gas gauge was just under 1/4 tank, I filled up and the gauge did not register full. after driving for approx 30mins the gauge slowly moved to full.

have the fuel sender unit checke( ground or loose wires) / replaced

Your fuel sending unit is probably a little sticky...
I wouldnt worry about it... It's the bottom half of the tank of gas that matters , right?

The float inside the fuel tank that measures how much fuel is in the tank needs to be replaced. The unit will register 8 OHMS if its any good off the wire leading from it when its at the full position

The sender unit in the tank is the most likely cause. To test the panel unit, remove the wire from the fuel tank unit and gound it. The panel gauge needle should move up full scale to hot. Remove the wire from the ground and the gauge should return to empty. If this test is ok, the tank unit needs to be replaced.

It sounds like the fuel sender is bad in the fuel tank.

the only thing that will cause this symptom is the fuel tank sending unit.

the fuel level sensor inside your gas tank might be defective
you need to drop the tank and check the level sensor

I had the same problem. My battery was going bad. When I replaced it with a new one, the fuel gauge worked fine.

The most likely culprit is the sending unit in the tank. They are relatively easy to change, as long as you don't have much fuel in the tank. It's a mechanical unit that has a float that raises and lowers with the fuel level, however with age, the floats can develop leaks and no longer float, of the electronic portion of the unit can go bad and not register the proper level. They are replaced and an entire unit, so it doesn't matter which happened, you will need to change the fuel tank sending unit.

your gas gauge sensor is most likley the problem on alot of new cars they measure your fuel off an average of were your float is at. like if your not moving it stays in one spot but as you move the float moves and reads the lowest amout that it is averaging and if your float keeps reading low your going to read that you have alot less gas than you really do. have you tried filling the tank all the way and see what it does? if it stays at full until you use some gas and your float starts moving your gas gauge will probley go down in that case its the fuel sensor if this dont help let me know and ill c what i can find out for you