Backwards. 73 empty and 10 full. Putts said grounding will peg the gauge, and that's true as it is zero resistance. So, to remember, the less resistance you have (more electrical power flowing) the more the needle will swing, and the higher the gauge will read. Therefore at 10 ohms it reads high (full). With greater resistance (less electrical power flow), the needle won't swing as far, and the gauge will read less. At 73 ohms it should read empty. HTHlike is it 10-73 ohm empty to full?
I would bet that the black prong must be grounded if you have a one wire gague as it uses the body for ground instead of a back wire to the gague!!