gas gauge

my 61 chevy gas gauge says full , when I know its not, just stared doing that, whats going on?

 

Grounding issue

 

Or stuck float. Disconnect wire from gauge or tank and see if it goes to empty.
If not I'd check the gauge. If it goes to empty then move to sender and ground.

 

If its showing full with the key on it may a bad sender. Take the sender wire lose. If the gauge shows empty it's the sender. Are the wire could have been pinched and is going to ground.