Modern Vespa

What is the resistance, measured on a fuel gauge sender unit (GTS250 ie.), empty/full? And does the resistance goes up or down when fuel level lowers?
My fuel gauge shows 6/8 when fuel tank is full, and when it shows 'empty', there is still 2/8 of fuel in the fuel tank. I would like to fix that, without replacing the sender unit or the fuel gauge unit. Replacing parts without the right diagnose, not for me

From the 2010 workshop manual, it states the following

Tank Full <= 7 Ohms
Empty Tank 90 +13/-3 Ohms

I just tested a spare sensor and measured
Full ~ 5 Ohms
Empty ~ 100 Ohms

I have a similar issue where the dashboard is always 1 tick short of full when full.

There is an adjustment screw of sorts at the bottom of the fuel sender unit, however I haven't yet tested if it affects the resistance values when adjusted.

Thanks a lot, that is something to work with. At this moment, I have 75 ohm in the 'almost empty' position, I'll try and drive untill the 100 ohm is reached, and compare the fuel gauge at that moment.
Adding an extra resistance will not help, while extra ohm means an even lower gauge...

If you have a 100 Ohm resistor you can put it across the connector(sorry don't have the wire colors handy) and see if your gauge show empty. Similarly, you can try a 5 Ohm resistor and check if your fuel gauge shows full.

I think the result will be the same if I fill her up and the sender resistance is at 5 ohm. And at 75 ohm, the gauge shows almost empty, so 100 ohm will show 'empty' for sure. I don't have the 100 and 5 ohm resistors available, but your solution is the way to go for testing the gauge. So when the sender values are ok, it must be the gauge, and I don't feel like replacing that part. Thanks for the advise, I'll do some test-work this weekend...