I have a 1973 Z and the fuel gauge is showing full, even though I emptied the tank and added only about 1 gal. This is a new fuel gauge component in the tank, swapped in in the last year. It seemed like it started having this problem after I put in a Pertronix Ignitor starter, but that doesn't seem to make sense. Here is a pic of the wiring at the tank, and 2 pics from ballast area under the hood. 

Make sure the fuel level in the tank isn't too high.

How to test:

1. Turn the key to ON. Use a piece of wire to short to the terminals at the sending unit while someone watches the gauge. The gauge should go up. Remove the wire after testing, keeping in mind that the gauge probably won't respond too quickly.
2. If the gauge does not respond in test 1, leave the key in ON. Use a voltmeter to measure voltage to ground at the yellow terminal. The voltage should be changing a lot. If it's not, it could be the gauge, but more on that later.
3. If the gauge moved in test 1, pull out the sending unit. Measure resistance between the two terminals while moving the float. If the resistance doesn't change, you have a bad sending unit.
4. If test 2 failed, double check at the body harness/dash harness interface. Find the yellow wire and test voltage to ground on the dash harness side with the key in ON.

Good luck and report back on the results. If you don't understand the tests as I laid out, let me know.

So, after I posted I went and switched the wires at the sending unit. Now it's showing closer to accurate. The gauge needle came up off the bottom of the gauge when I turned on the car and settles the needle right at E, which is about right. I've since added 3-4 gallons and the needle only moved up a little, like less than an 1/8 tank. So, I'll fill it up and see what it shows, but I think I'm back to how it was last year, showing less than is actually in the tank. How do I adjust that? Tinker with the sending unit?

Btw, Test 1 worked. It slowly moved the needle to almost full. 

As the float rises, the resistance drops. If you are sure the sending unit is registering low, you could bend the arm down on the sending unit. I would go slow on that, though.

1 hour ago, Dadsun said:

So, after I posted I went and switched the wires at the sending unit. Now it's showing closer to accurate. The gauge needle came up off the bottom of the gauge when I turned on the car and settles the needle right at E, which is about right. I've since added 3-4 gallons and the needle only moved up a little, like less than an 1/8 tank. So, I'll fill it up and see what it shows, but I think I'm back to how it was last year, showing less than is actually in the tank. How do I adjust that? Tinker with the sending unit?

You're not going to like the actual fix: There are adjusters on the back of the gauge for both low and high ends of the needle. Ya, back of the gauge - so getting your fuel gauge to actually represent something close to the amount of fuel in the tank would require removal of the gauge. Then you need to empty the tank and set the "E". Then you have to fill the tank and set the "F". Then you have to reinstall the gauge. 

In 10-20 years you'll have to do it again because the resistance of your wire and connectors will have changed... And that's why my fuel gauge shows 7/8 full when absolutely brimming with fuel and WAY to the left of "E" when my fuel pump starts sucking air.

Well, that's depressing.

If you hit the rubber grommets with a heat gun (or the wife’s hair drier) it’s gets soft enough to pull over the terminals.

Edited April 28, 20231 yr by heyitsrama

One other thing to note is that due to the shape of the tank the gauge comes off "F" fairly quick because the top part of the tank is way smaller than the bottom part of the tank. The fuel gauge moves much slower in the bottom half of the gauge.