I'm also wondering what the fuel gauge is doing in my B13 Nx. I just replaced it for a know good one because the one I had was quite off, would seem stuck, pointin a little low, dropping soon, not dropping for 70 miles and that was obviously the sensor in the tank.
But still, I can agree the gauge seems to drop fast the first half and the other half seems to last longer. I also have issues with resetted ECU's (and there are different ways of resetting just one really works, leaving the car overnight without battery, bc I learned 20 minutes just empties error memory leaves issues like my ticking AAC and then I left it battery less for 2 hours and ticking was gone, which I wasn't able to before.
To keep the long story short, I don;t even trust the ECU no more, after a good reset tank seems to last longer. Then when I fill it up to max, mileage seems to suffer. Yet, I can't proof this.
Also be sure to fill the tank way up, and use the trip meter for good measurement, so you need to squeeze in that last liters where it overflows all the time. Else you cant be sure you filled to the same level, even with same gasstation and pump.