I think you ought to stop gazing at your fuel gauge. They're not very reliable at the best of times. For the first 100 miles on the Cit.BX that I used to drive, the fuel gauge hardly moved, then over the rest of the scale, it moved quickly. You should brim the tank, make a note of the mileage, drive for 300 miles, brim the tank again, then divide the miles by the fuel bought.
Incidentally, my daughter drives a 1.6 C4 and after driving a 2ltr Xsara for 12 years, her comment was that she hardly ever puts fuel in it.