I put a new tank and fuel sending unit in a 69 Firebird a few weeks ago. I always hook up the sending unit before i bolt everything back in the car to test that the sending unit is working and the gauge is accurate. So with the float all the way down the gauge reads empty, then i move the float all the way up and the gauge reads full.
I put everything back in the car and put gasoline in the tank and everything is fine.
The owner had the car for a few weeks and was saying he put 13 gallons in and it was full. The tank is a 18 gallon tank.
I get the car back and drain out 5 gallons then check what the gauge says, it was on half a tank, i drain out 6 more gallons and the gauge was on empty.
Something is definitely wrong.
I pull the tank back out and there is still another 4 to 5 gallons remaining in the tank. Meanwhile the gauge was reading empty.
I take the sender out and start measuring the depth the sender sits at and notice that the pick up tube is about an inch and a half from the bottom of the tank and the float it a little above that.
So with these sending units they are selling now fuel readings are not accurate.
This is why the customer was able to only put 13 gallons in and says it was full. the whole range of the float lever is off and the readings are not true to how much fuel is exactly in the tank.
Ordered another fuel sender unit. Hopefully it will be different.

So after 3 different sending units it's apparent that there is little to no quality control on these units. At a quick glance they look the same, but when you compare them side by side they are bent differently and one that was a bit longer to reach closer to the bottom of the tank. Then checking them with an Ohm meter one of them was reading all over the place and was not steady. The one that i finally put in the tank had to be modified by trimming the tab that stops the float arm in the empty position so that it would read the proper 0 ohms with the float arm all the way down.
Put everything back together and with the tank back in the car, started filling the tank 5 gallons at a time and checking the fuel gauge. So after completely filling the tank the gauge seemed to correspond with the actual amount of gasoline in the tank.
Sent the car on its way.