Don't trust the fuel gauge (no that linear) or the computer. You need to measure the fuel going in to the tank v miles/KM done.
Fill the tank, drive it, then fill it again and work out your MPG or KM/L. I normally get ~19 MPG or 26 on a long run (US gallon 3.78L). So by my maths thats about 8 KM/L normally.
From what I've seen here I would say my car is fairly representative of what a 540i should do in good order.