Gas gauge displaying improper level

Hey all,
First post here, lurked for ages.  I have a JA22W and I have been having a gas gauge issue.  No matter what I have done, the gauge reads 1/3 when full and then drops to 0 as the fuel runs low.  The gauge never hits full.  
I have replaced the sending unit in the tank, and today I pulled the dash and put in a whole new gas/temp gauge.  Both of those didn’t fix the issue.  Any thought on how I should proceed?
any help/guidance would be appreciated.

Are you getting a full 12V to the sender?  There may be a high resistance in the wiring.

When you had the sender out of the tank did moving the float manually make a difference to the gauge? I'm wondering if the float is intact and or if the arm needs bending/calibration to match the limits of the tank?

I don't really know how they work. The one in my Gen 3 used to read the last level with the ignition off, and would only change when the ignition was on, which indicates a moving iron type, but the response is very slow and the diagrams indicate it has a regulator inside, which would be bimetallic- I've never pulled a gauge apart to see.



Anyway, as you have changed both ends, and the regulator seems to be part of the fuel gauge, I'd check the resistance of the tank unit when it is in the car, in case there is some obstruction in the tank stopping the float rising (or it is fitted in the wrong orientation and the float is hitting the side or a baffle).

I'd check the spare first, just move the arm and check the resistance against the table (to see if the table is the right one, I only have a leaf spring manual. Then check what the resistance is as you fill the tank up, if it reads 1/3 the resistance must be 40-50 ohms.



If the resistance of the sender is right, I'd jump a temporary wire from the sender terminal back to the gauge yellow/red wire in case it is a fault with the wire- also check for corrosion at the plug terminal for that yellow/red wire.