Gas gauge 1/2 full

[truckin 79]

So my gas gauge reads 1/2 full and I am pretty sure it is full. I tested gauge with 9 volt battery and swings to full and empty when applied. I ohm tested and it read 90 ohms between top posts. I have 2 old gas gauges and they read about 45-50. I thought I would switch resistors and would fix problem but hit 1/2 tank again after swap. I can move needle to empty or full add power and still moves back to 1/2. What is my next move? Thanks