Regardless of what your fuel gauge displays, it would be much more logical to reset your trip meter next time you fill up so you get more accurate data.
Then when you fill up AFTER that, you'll know how much fuel you used and how many kilometres you drove.
That's much more useful information (eg 11L/100M) than "I get 400km on a tank".
You may find that you are only filling 60 litres of an 80 litre tank and are getting the same fuel economy as everyone else.