Fuel Gauge not accurate.

My fuel gauge. A bit hard to explain but I filled it up and my fuel gauge went to the top. After doing some driving I watched my fuel gauge slowly reach the little orange mark and go beyond that. So I filled up again believing I had chugged through that whole tank but to my disbelief I had only used roughly 10 liters of petrol. I have been told the tank hold 40 liters so this leads me to believe my fuel gauge isn't registering the bottom 30 liters of my tank which makes fill up stops hard to calculate. Any thoughts on how to fix this? Been thinking of just putting in an after market fuel gauge and hoping that fixes it?  :dontknow:

sounds liek the sender to me, its fitted into the top of the tank and is accessible after removing the round black disc under the rear seat

Okay then. I'll have a look as soon as possible. I'll let you know how it goes…