If I were you I'd do this:
First, with the sender OUT of the tank, hook up a good ground wire to its body
and connect the regular wire to it.
With ignition ON, let the sender all the way down, wait a bit and read the gauge.
If it shows empty, or bottom of R, that's good.
Then move the arm all the way UP and wait a bit and read the gauge. Should read full.
If you can, bend the arm so the float just sits on the bottom (empty tank) when
all the way down. You can do this by measuring the distance in the tank.
If you have the right sender, that should calibrate it fairly well.
It may be necessary to run an auxiliary ground wire from the sender to a tank mounting screw.
I have had senders where I had to do that - apparently the tank was too well insulated
from the car.
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George Downs Bartlesville, Oklahoma