Why do you believe there is air space at the top of the tank? IF the filler neck on say a 68-70 Charger is full of fuel, how could there be an air space ?

Whether the neck is full or not, there's still going to be air pockets in the tank. Even the vent lines aren't completely against the top of the tank.

Either way, the test is simple enough to try.

I've had nothing but problems filling older cars with gas and the spill over. I don't know if it's in the design of newer pump nozzles or what, but now I put my ear down and listen for it getting full and stop manually.

It's the curse of the A-bodies - learned long ago to listen to the rising tide of gas...

they do work if you're fairly close (within 1/4 tank). I've installed quite a few as it sure beats filling, emptying, tweaking the float arm and repeating with no gain.
A bit of a PITA due the wiring etc. but less so than the mentioned circle jerk

After weeks of messing with this I ended up buying a new Auto Meter fuel gauge and Auto Meter fuel bridge to try out (pic). Couldn't get the fuel bridge to work as it should so I hooked the gauge up directly to my sender. The gauge now reads the sender but it reads backwards, full when empty and empty when full. At least now I know when it's empty or full just have to remember it's backwards. I can live with that.

I use mopar filler neck grommets, lock rings and sending unit seals/gaskets. I tried aftermarket ones and I had a problem with sealing. The aftermarket lock rings are crap too.

Yep, they are thinner than the OEs.

Joe

i got a reproduction tank filler tube grommet from a reputable supplier [can't remember who though] for my charger, and although it fit my tank perfectly, i couldn't get the filler tube through it to save my life !
so i got another one from someone else, and it fit perfectly. may have been a genuine mopar item, but i can't remember.

Not sure how that's wired but typically a DC circuit will indicate backwards if the polarity is reversed. So you might want to double check the wiring

Yeah, that's what I was thinking, already tried that, same problem. The Auto Meter instructions say if the gauge reads backwards switching wires won't fix, need to buy a sender that's compatible with their gauge. I'm on my 3rd sender, I'll just live with what I have now. Tired of dropping the tank for this. Plus no one can seem to build a sender compatible for Mopar gauges, how in the world do we find a sender that will fit in an A body Mopar and be compatible with Auto Meter gauge? Good luck finding one of those, probably not even made.

I went through quite a circle jerk with them several years back on a VERY expensive set of programmable digital gauges in a customers car. they said they no longer have engineering as it is all done outside. I did what one is not supposed to and changed 3 things at the same time. I don't know to this day what fixed the issue we were having as it magically just started working. After losing 60+ hours on the problem I wasn't going to backtrack and try to identify the root cause. Still have a bunch of components that they kept sending ( throw parts at it) They are not the company they once were

I just did a quick scan on the instructions and it should work I'll repeat, They are not the company they once were

okay, there are 3 prongs on the gauge; sender, ignition and ground, what wires do I switch? The instruction clearly state; swapping wires will not solve the problem.

Short answer is NO.
There used to be someone that rebuilt OEM senders. I don't know if he is still around.

Saw that, but they also claim it will work with any sender or you can do a custom calibration As I mentioned they are not the company they once were.
out of curiosity did you try # 6 on the list ?

One could try swapping the 12V source and gauge ground but if they say that won't work ask the [censored] will ???

Yeah, I tried every one of those, couldn't get any of them to work, so I hooked the gauge direct to the sender and it reads the full range but backwards. Might just install the gauge upside down, at least it will read as empty on the left and full on the right side like a factory gauge. Thanks for all the input. What makes me mad is Chrysler made a sender that works, why can't aftermarket reverse engineer Mopar's sender and make one that reads correct? Guess the engineers back in the day were a lot smarted than this new crop of engineers.

back in my working career as a machinist/shift supervisor, i can't count how many times i argued with company engineers on all kinds of topics, including sequence of machining operations, tolerances, measuring techniques, etc.
in case you don't know, engineers are the smartest people on earth. just ask 'em...........

per there diagram and instructions there is no reason it shouldn't work correctly. They even show the sending unit as working off resistance to ground. Providing your sending unit is properly grounded (worth a check) which you've likely already done.

One could try Running a separate jumper from the battery negative to both the gauge and sending unit. doesn't have to be more than 16 ga, (18 would probably work).
If that doesn't work try a jumper from the Positive post to the gauge.
There is also mention of a "Custom calibration routine" when the S/U resistance is unknown. not sure if you tried that or not.
Still reads backwards? You have a bad gauge or bridge PERIOD!!!!. This is not rocket science. They are BS'lng you

if so inclined one can even use 0-100 ohm rheostat in place of the sending unit. bet it still reads backwards.
P's me off what our support systems have evolved into (worn out jock straps).

I'm going to assume that at least your not talking to a foreign country YET!!!

As the son of an engineer, can confirm.

15 years after his death, I'm still cleaning up his mistakes.

Another question just occurred to me,
referring to their chart pictured in a previous post does their first number refer to a full or empty tank. I would ASSume empty BUT?

Form tanks website:
The resistance of your fuel sender should match that of your gauge. For instance the resistance of the stock gauge in a 55 Chevy has a value of zero ohms when empty and 30 ohms when full. The following list is for stock gauges and some popular aftermarket brands:

Ford up to 1986 - 73-10 Ohms
Ford 1987 & up - 16-158 Ohms
GM up to 1964 - 0-30 Ohms
GM 1965-1997 - 0-90 Ohms
GM 1998 & up - 40-250 Ohms
Mopar up to 1986 - 73-10 Ohms
AMC 1950-1977 - 73-10 Ohms
Autometer -240-33 Ohms is the most common however other ohm ranges are made
Classic Instruments - 240-33 Ohms (excluding vehicle specific gauge kits which use factory ohm range)
Dolphin - 0-90 Ohms
Dakota Digital - Programmable to work with most Ohm range senders
VDO - 10-180 Ohms
The first number represents the empty Ohm reading and the second number is the full reading.

IF their chart is reverse of the above, that would cause the problem.
IF your sending units resistance board was installed upside down, THAT would also cause the problem. Easily verified by checking the readings at full and near empty.

You can also verify the grounds a suggested earlier.

last on their list is a 0-1000 ohm custom calibration that should correct the issue regardless, providing the GROUNDS are good. Keep us posted