Fuel sender unit, its a cr@p design, The measuring device is a flexy piece of metal that rubs along metal contact points, obviously this wears overtime or can get dirty. Its encorporated with the fuel pump located behind the Sub. Mine wouldnt read above half at one stage. replacement of this instantly cured it!
The only bad point is because the fuel tank goes over the prop shaft (cant think of the term now) its n shapped therefore there is another fuel sender on the oposite side under the secret cubby in the passengers glove box.
Any more info, drop us a PM.