Only real way to test is to fill tanks, take note of how much fuel went in for how many km's over quite a number of tanks.
If it's only over a shorter distance then any variables are going to be exaggerated. (e.g. 6 uphill, 4 downhill trips on one record vs 6 downhill, 4 uphill on the next one - all due to which part of the journey you reset the counter)
I don't think??? the levels in the fuel tank come into the consumption calculations? Just the distance to empty (which is based on the level of fuel and the recent consumption)