Problem with fuel gauge., Shows only 1/4 full. Options

[daytona]

Hi world,
I need some help with the fuel gauge in my 74. The car is a 6 cylinder conversion but I don't think that this factor contributes to the problem.
The issue is that the gauge shows that the tank is 1/4 full even though it is full to the top.
I have two other gauges that I tried and both show the same 1/4 tank level. In my book this would indicate that the sending unit is the culprit. Couldn't be that all three gauges have the same defect. However, measuring the voltage at the gauge, I get 12.4VDC. I think this means that the sending unit is sending the correct signal. So where is the problem???
Has anyone had this problem? Any ideas?
Any and all help is appreciated.
Thanks,
Bill.

[Tbrown4x4]

Some of your terminology is off, so hopefully this helps:

The gauge should have battery voltage to it, and the sending unit provides the ground circuit. It has a float that rides up and down on a guide rod. The float has contacts (wipers) that run on resistor wires in the sending unit.
When the fuel level is high, the resistance is low. More current flows through the fuel gauge, and it shows a high fuel level. When the fuel level is low, there is more resistance, so more less current flows through the gauge, and it shows a low level.

Like George said, the most likely problem with a gauge reading low is the float stuck on the guide rod. Mine would stick on 3/4 full until I hit a bump, then it would read correctly for a while.

These cars can sit, and condensation in the fuel tank causes the rod to rust and keep the float from moving freely.

Take it apart and clean it. Just be careful of the resistor wires. They are very fragile. And replace the gaskets.