Parthery said:
Check all your wiring first. A loose ground wire on the sender will show an empty tank.
If the wiring checks out and the sender is bad, call Grady and get the tank depth. Then order a custom one from WEMA. Website is wemausa.com. Sender should be 1/2 inch shorter than the tank depth.
If the sender wire from the gauge is disconnected at the gauge or at the sender, the gauge will read empty.
To test , connect the wire at the sender ( the wire from the helm, its either purple or pink, I can never remember) to a good ground. With the ignition on, the gauge should read full. If it does, the wiring to the helm and the gauge are good.
As mentioned, the tank ground could also be bad. If the above test works, connect the ground tab on the sender to a good ground. If the gauge reads whatever the tank is holding, the tank ground is bad