xp9fa d-4d 2008 Fuel indication stuck

[Coldhammer]

Specifications:
Toyota yaris xp9f (a)
Engine: Diesel d-4d 1'364cc 90bhp
Tank: 42 litters capacity

Error: Fuel gauge is propably stuck

Explanation:

Well i filled the car with diesel yesterday and everything was fine!

So i did 110km and I must say that I pressed the gas much more than usual
to 140km/h so as I expected my l/100km hit at 5.3

And of course I lost 1 out of 8 lines of fuel... pretty normal.

anyway because i don't like the 5.3 number... i decided to fill up again and reset it and so I did. And for my surprise the fuel gauge was still at 7 out of 8... anyway I tried filling it to the maximum "be able to see diesel in the pipe" but still 7/8.

Can anyone explain why and how to fix it?