Working on a friends' '67 Coronet, about 8 months ago I replaced the original fuel tank with a new stainless tank. I replaced the sending unit several years before with a stainless unit. After trying a 7" ground clamp and then a wire clamped to the outlet tube to the body, the needle only moves about 1/16". I grounded the wire from the sending unit to the body and the gauge goes to full. Also, the fuel line to the pump is stainless.
Is there problem using a stainless tank that should be grounded differently?
Thanks, I hope I covered everything,
Tony

Remove the sending unit so you can move the float up and down and use a good volt ohm meter to see how much ohms the sender has at the lowest and highest lift from the float, I'm thinking from 15 ohms to 30 or maybe it is 15 to 60 ohms, CRS
If it doesn't change you need a better sender
if it does change then follow the wires to where it doesn't change going to the gauge
Gas senders are wire wound variable resistors, also known as varistors, there is a lever on the float arm that rides against the wires to complete the circuit back to the gauge changing the resistance as they go up or down

I've got the same problem here with a steel tank and a SS sender. I have a wire clamped to the sender with a ring terminal on the other end screwed into the body, and the sender has continuity with the body. I tried to solder the ground wire to the sender, but it's SS so that worked about as well as you'd expect.

At one point I pulled the sender and checked it, rang the wire, and grounded it to see the gauge go to full. All those tests passed. The temperature gauge works and the IVR was confirmed to work. At the time the tank was pretty much empty but I put a camera on the gauge and watched it move a little bit and thought I had it fixed until I filled the tank and the gauge still moved just a little bit.

I'm interested to see what suggestions are made to your problem, with hopes they will help me too!!

A wire wound resistor is not varistor, it is a wire wound resistor. A varistor is an electronic component with an electrical resistance that varies with the applied voltage. A wire wound resistor is always the same regardless of voltage applied.

10-73 ohms is the specification for the sending unit's resistance. Pull the sender, verify that measurement, look for a smooth transition as you sweep the float arm from full to empty.

There is nothing special that need to be done to ground the tank just because it is made of stainless.

I had a defective aftermarket stainless sender in my charger. It never read correctly. I ended up finding a NOS one.

A simple Ohm's test would tell you if its grounded. Sounds like a improperly installed or screwed up sender.

The meter, if hooked between the signal terminal and sending unit flange/tube,
Should change while filling the tank. This will confirm the sending unit is the problem as it likely is. Welcome to the world of aftermarket crap.

If you look at a stock sender and figure out how it works the sender resistor has exposed wires for the sending unit tab to ride on as it goes up and up and down so it varies the resistance

A varistor varies it's resistance based on how much voltage is applied to it.

The sender is nothing more than a rheostat that uses a wire wound resistor, just light the dash light dimmer. IT IS NOT A VARISTOR.

This is what a varistor looks like, if you have one of those in your gas tank someone F'd up.

As you already know the gas gauge sender does not receive voltage form the gauge, the gauge reads the ohms from it, no voltage

You really need to stop commenting on electrical things, you know nothing about them.

The fuel gauge CIRCUIT consists of the instrument voltage regulator, the fuel gauge, the fuel sending unit, various wires and the ground.

How the voltage flows in this setup is from ground, thru the sending unit, to the gauge, to the IVR and back to ground.

Assuming the system is working properly, if you put a meter on the fuel sending unit connection, without disconnecting anything, turn the key on and you will read a voltage there. That voltage will vary depending on the fuel level.

That is how an electrical circuit works.

Read the ohms of the sender through a full sweep of the arm & take note, then reconnect to the fuel gauge wiring & read the ohms at the fuel gauge connection, should be relatively the same, if not u have a problem with the wire or the body ground path.

Have you tested any Mopar gas tank circuits that way?
If not, maybe you should.
You do understand the difference between ohms, volts and amps, correct?
Can you imagine a spark occurring from the gas level sending unit in the tank on a hot day when the car hits a big bump making the lever jounce around with less than a full tank of fuel? work:
As far as my experience and understanding of electrical theory on both A.C. and D.C. circuit I do understand a smidgeon about those circuits and application,
I spent 33 years fixing telephones and trouble shooting the network which help me learn about circuits and the difference between Volts, Amps and Ohms and how they work
Earning an Associate of Science in Electronics degree and extensive classes on computer hardware and software helps also

It's been a while, I replaced the 2 year old sending unit with a new sender. The gauge actually shows movement and shows 7/8 full with a full tank.

Thanks to all who provided suggestions.

Mopar gas gauges operate at a certain ohm range and you must get a sending unit that has the same ohm range. I don't remember what Mopar's ohm range is, you can search and look it up. When you purchase a sending unit they should tell you the ohm range of the sender so get the one that matches the Mopar gauge ohm range. If the vender doesn't list their ohm range for their sender keep looking for one that does.

Every sending unit in every gas tank, fuel tanks, built operates by varying the ohms in the senders, in airplanes, boats, cars trucks and stationary internal combustion engines with liquid fuel gas tanks that use remote fuel level meters, gas gauges: scope :

A typical Mopar sending unit's resistance varies between 10 & 70 ohms (+/-)

your experience is typical with replacement aftermarket fuel sending units across the board. Spectre started out trying to correct the issue but has failed. Trust mo on this as we have spent HOURS trying to make them read correctly on numerous vehicles.
IMO you have two choices.
1. live with it knowing that 7/8 is full and hoping that E leaves a couples of gallons of reserve
or
2. Buy a device from tanks that allows you to compensate for the S/units discrepancies. Not too bad to install and fixes the issue. certainly less time consuming than trying to make the units read correctly.

TANKS SENDING UNIT FIX

those Meter Match devices work great.
when it comes to new sending units, i just bend the float arm around so it reads full, then see what it reads when it drops to the bottom of it's stroke.
i then make a note of what the gauge reads, and fill up before then.
i usually keep the tank mostly full, and any more, i don't venture too far from home anyway.