Or to put it another way, your assumption is that they all read 0% at the bottom of the sender. They don't. They all read 0% at the same height down the sender.
Yours and Charles assumption is that the resistance wire has exactly the same properties (not necessarily gauge) from one sender length to another. Since VDO offer senders in the range 150mm to 800mm length but with a much lesser resistance range, it suggests differing wire resistivity is spec'd through the range.
Add to the above the findings by planenut that the two senders in question measure a similar resistance. Consequently both he and seemingly most others that fitted the 280mm sender, including myself, noticed an increase in fuel indication immediately after fitting. Not a perfect solution but a useful increase in mileage through the indicated range.
This image shows how the sender works with two parallel resistance wires connected to each of the ground and signal terminals at the top but isolated at the bottom. These in turn being shorted together at any point between upper and lower limits by contacts on the float, and so giving a linear varying resistance.
Richard
2018 Roadster 3.7 1966 Land Rover S2a 88 2024 Royal Enfield Guerrilla 450 1945 Guzzi Airone
