The tank floater can lie about the real content of the tank if it is bend and not anymore in the original curve, or if the floating piece has some liquid absorbed in the years and is not empty itself which causes it to sink deeper and giving on the fuelmeter the idea there is less fuel in the tank than in reality. (Anyway, the second situation is the case with mine.)
Guido
The original fuel tank sender and tank could never show the true amount of fuel in the tank because the top of the tank has a much wider and larger cross section than the lower part, but the sender rheostat is linear.
So the fuel will drop slowly when it is still in the larger top section than when it is in the lower narrow section. This has all been explained in my technical article on the fuel system which has been on my website in the technical article section for some years.
I stated that if you were on a long jouney with a full tank, you might get 340 kms whilst the gauge dropped slowly for the first 'half' but then only get 200 kms during the second 'half' from half way to almost empty. This is due to this linear sender but irregular shaped tank.
The other problem is that all the Murena I have ever come across have a sender that has the float too high at it's minimum, so it gives you a false early warning that you are almost out of fuel. I have reset all the senders in the Murena I have owned so they are more accurate in telling you when you are almost out of fuel, and do not give that flashing low fuel warning light, much too early. Consequently I can get as much as 550 kms safely on a full tank before I need to refill with fuel even at my fast pace, and have done so many times. When driven at no more than legal speed limits in the U.K. and France I have actually achieved 700 kms once or twice. This is with a 2.2 Murena with a Holbay cam and 140 bhp!
Anyway, please download my technical article (if you haven't already) on the fuel tank and system to find out more. It might surprise you.
If you really want the fuel gauge to give you a more accurate reading all the way from full to empty, then there is an electronic device you can fit and set up so it will be accurate at full, 3/4 full, 1/2 full, 1/4 full and empty. Contact me (at my domain email not here) if you wish to know more.
Roy