The primary variable left is to determine if the float is at the lowest limit of travel when it is installed in the tank and the resistance at the position.
What resistance does the sending unit read when it it installed in the tank and the tank is empty?
What does the gauge read when you have the sending unit out of the tank and you let the float drop as far as it can go?
I agree with the suggestion to look at the position of the wiper on the coil.
