Is there a fuse for the gas gauge in a 1966 Valiant

Then the fuel gauge is getting power.

You can do the following to check if the gauge is operational

With the key in your pocket. (That way the key is for sure off)

Take a jumper wire and go from the sender wire to ground at the tank. A good ground will be on the body somewhere preferably not painted or too rusted.

No get in the car and turn the key to run

Watch the gas guage, it should rise rapidly to full. Turn the key off before it gets to full. OR YOU CAN BURN OUT THE GUAGE!

It that works as described, you have proven the gauge, the wiring to the guage is good so the problem must be the sender
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That can be checked in car with an ohm meter. Go between the post on the sender and ground.

You should get between 10 and 74 ohms depending on how much gas is in the tank.