Gas tank real capacity? Is there an amount before gauge will register?

[oldchevelle541]

The sender may be junk, it may just need adjustment.
Note the A body sending unit below. First picture shows how it was right out of the packaging:

View attachment 1988116

The float and arm sometimes are too short and don't allow the float to rest on the floor. With it set as it is here, I could have 2 gallons or more before the float actually moves. You'd need a multimeter to determine what a few degrees of movement to the float arm actually do to the OHM reading.

View attachment 1988117

Without extending the float arm, I don't know a way to alter the readings the gauge will display. One could weld or solder an extension to the arm and reshape it. I have not experimented with it to find out but hey....I'm retired so one day I might. I could do this on a bench using water instead of gasoline. Maybe I'll see over the summer.
Back on point....
The filter sock in the above picture would have allowed the engine to starve and stall with maybe 2 gallons of gas still in the tank. I bent the tube until it touched the floor and was happy with it there instead.
Now, In 2002 I installed a 3/8" sending unit and new tank in my red car. It was never that accurate. With gas spilling out the filler neck, at best it read just shy of 3/4 and dropped from there quickly. The 68-70 has a 19 gallon tank so technically, 4 3/4 gallons from empty should register as 1/4 on the gauge. No. Mine was pretty much always reading at least 1/4 below actual. 4 3/4 gallons....no needle movement. Half tank...maybe it would read 1/4. 14 1/4 gallons should read 3/4 but was now around half and yeah.... full tank was maybe 3/4 on the gauge.
In 2015 I converted to an aftermarket instrument panel, all digital components. Part of the calibration process involved mapping the fuel sender curve. I wanted it right so I pulled the sender and bent the inlet tube like I did in the A body tank shown above until it rested on the floor. It turns out that it was about 1/2" higher than it should be so it too allowed the engine to run out with about 2 gallons of fuel still in the tank.
After calibration, it does read full with a full tank but it still is not linear. If I get 240 miles to a tank, it isn't like every 60 miles I see it drop 1/4. Unfortunately, I use the gauge as a guideline, not "gospel".

This is good stuff but reading everyone's issues before and after getting a new sending unit you wonder why many worry about getting a new unit and from whom. Mancini has on their website that their units are calibrated to work with original wiring. Has anyone purchased one. They have non-stainless units that are very reasonbly priced.

 

[Kern Dog]

Well, all seem to work but they are not accurate like a factory one. Any company can creatively write a description like that and technically be true.

 

[RJ Squirrel]

Any vendor selling aftermarket senders claiming they "work" should show a picture of the resistor board that needs to be tapered to
mimic the shape of the fuel tank, as I learned yesterday getting myself educated up on these rascals.

 

[RJ Squirrel]

After I was succesful with the first sender, I went out to the scrap heap and found the tank from the Roadrunner I took out 3 years ago, the sender still in it. I cleaned it up and fixed it too. If the resistor board is still intact they can be saved. Mine both needed the connections to the board and the wire passing thru the end plate replaced. I am putting it back in the RR, the aftermarket one has never worked right.

 

[oldchevelle541]

Fully agree with all recent comments. Thats why I hesitate to buy anytihng. You would think with all the press on replacements not being accurate some company would see an opportunity and step up and make these things truly perform like the originals.