Fuel Gauge help - VR6

It has always been a problem but was low on my things to do, but now im getting short on readies and the next big step is financially some way off i think its a problem i may tackle soon. Basically i fill up and my gauge only goes to 3/4 full. I ran the car on red for a while to see how much was still in it but then got scared id run dry so stuck 30 quid in which only filled it 1/4 full. Any ideas what causes this? Is this repairable?

tah yah

Ah
So you are saying that a 1/4 of a tank, say 30l equates to a 1/4 fill on the guage and that full equates to 3/4 on the guage. So asumming it is about a 60l tank then it would appear to under read by 1/4....
Interesting.

Now where is the tank and what sort of sensor does it have? It might have a rod like a ballcock that is mangled.