You need to check the actual resistance of the sender with a known quantity of fuel in the tank. Anything less than full is difficult to know exactly how much unless you drain/refill with a know quantity. It should check pretty close to 10 ohms full - start there. If not then you will need to pull it out and see if the float has failed or the arm is bent wrong or the variable resistor is failing, etc. Truth is that many of these senders on the market today are poorly manufactured and are junk.