Another gauge corrector--cheaper than MeterMatch, 33.00
Your numbers are quite a bit different than mine, which are shown in the picture of my calibration box. This is like due to your 2 gallon offset.
I think the difference in numbers is due to the junk they are selling as a replacement part. The empty point I am using is certainly a part of it as that would move all the other points up, but the full mark is off as well which can only be a result of the sender. It is possible that if I dropped the tank and took a look at where the float sat when the tank was full, that bending the arm could adjust some or all of that difference out, but I'm not going to go through the effort. My gut says it isn't an issue with the float travel but instead with the shoddy part I got from the manufacturer.
I will say, if I end of ever replacing this one with something similar I will probably take some measurements before I put it in to see if there is something that could be done to make it better.
In the end, I plan to drop the whole setup though. A couple of years ago a friend helped me modify a tank to use a gen5 Camaro pump and basket for an EFI swap. When I get to that point, I will probably just use the GM sending unit and this Fuel Bridge to keep my stock gauge.
The glitch at 12 is interesting, I assume the red & blue lines represent two different fills? If these are two different fills and it repeats, it could be a fault in the sender.
Not two different fills, just two different readings. I got concerned later in the process that the fuel could be sloshing and cause the reading to be off, so I started taking a reading right after I did a fill and then just before I did the next fill. This gave it time to calm down while I refilled the 1 gallon gas can. I did get slightly different readings at points, but the one at 12 gallons had way more skew than any other point did and was way off for what it should have been. Not sure what is going on there. I should add that this was with a cheap HF multimeter so there is always the possibility that my tools are the problem as well.
Now that I think about it, it does seem like the gauge would drop really fast at one point and then come back up. If so, it would kind of match the readings and indicate that the sending unit has some kind of flaw at that point.