Fuel Guage Issues

[T2vigil]

First I have a 64 Chevy C10 with guages not the idiot lights. I aquired a guage cluster from what I assumed to be a 66. It had guages along with a tach. In rebuilding my dash I swapped the fuel guages. The old fuel guage was reading just under 1/2 full. Now the swapped fuel guage reads almost full. Does anyone have any ideas why they would read so differently? FYI I checked both of them with an ohm meter and they read 38 to 40 ohms.