[rick55]

The easy way to make sure what is going on is to remove the sender from the tank. Connect the feed from the gauge to the sender and connect an earth to the mounting plate of the sender unit. Turn the ignition on and move the sender through its travel to see what the gauge is doing. If it works correctly it is an earth problem on the tank which you can fix by connecting an earth to one of the sender mounting screws and screw it to a clean metal surface in the boot. It could also indicate that your float is being restricted in travel.
If it still behaves the same you have the wrong sender.
The problem someone else has mentioned about needing to remove all the screws for the gauge to work correctly suggests he has a sender unit of the incorrect value. Wit the unit unearthed they will find a path back to the battery however they can. What this guy is doing is increasing the resistance in the circuit.
Your problem sounds like your sender resistance is too high.
You may just simply have to bend the arm of the sender so it hits the bottom of the tank.