Hi Guys,
As already said the 100 liter is about a couple of inches taller than the standard and both of them have 3 thin wires and a floater inside the aluminum tube. If the section of the tube is not round and the wires are in any way damaged, the sender will not give the right reading at the instrument cluster. Repairing it is not easy but possible.
What i want to say to anybody is looking for 100 liter tank that matches the original porsche specs for this option should also need the correct 100 liter instrument cluster. Have a look to the PET and, for example, for 1968, 90174150207 is the ''80 mm instrument cluster for 100 L fuel tank'', 90174150205 is the ''80 mm intrument cluster for 62 L fuel tank''. I have both and the only difference that i noticed is the PET number on top of the instrument, behind the glass, but for sure there is some internal difference.
Said that be sure to have at least a tall working sender. Original wiring is fine as it is. Don't mess with it.