I agree mate.
unfortunately they all use a float-driven variable resistor so "level" is still "level", but under a constant flow, the resistance values of the sender as it goes from full to empty can only follow the top surface of the fuel, and as you can see a round bottomed tank will not give a linear reading from top to bottom as the curved lower part has less volume.
I would have thought a better idea would be a load sensor / strain-gauge that measured the
weight of the fuel in the tank - and could exactly display the tank contents to the millilitre as fuel has a known weight, like water does, due to its specific gravity value.
The strain gauge could be mounted in a small depression in the bottom centre of the tank so it gets a pretty constant reading.
