If you think about the shape of the fuel tank (basically a cylinder on its side), the result is a rather non-linear fuel measurement. When full or very low, the level drops quickly. When in the middle range, the level changes very slowly compared to the fuel used. Even a carefully calibrated fuel gauge is not going to be particularly accurate under those circumstances. As I learned in flight training, never trust the fuel gauge.

I note that when my tank is full, the gauge pointer will read just above the full mark on the scale. Unfortunately, there isn't a pin-rest so when the tank is less than full but above about 2/3 full, the sloshing in the tank (due to movement) will cause the pointer to swing way past the full zone and then stick there. If the vehicle is at a full stop for several minutes, the gauge may swing to a more expected reading ... or it might require a bit of tapping on the gauge to get it to un-stick. 

If you were to replace the gauge, I suggest you try to get one that has a pin to keep the pointer from wildly swinging around (and possibly sticking at the extreme end of the travel). However, due to the shape of the tank, I doubt that you'll get much more than a rough indication of fuel quantity with anything more than +/- 10 to 15% accuracy.