Fuel gauge reading 100% full no matter what

I recently built a 24 gallon fuel cell using the factory fuel pump assembly from a GMC Jimmy 2 door. Factory fuel tank was ~18 gallons bone dry. I changed the value in the tune (PCM was from a Suburban 5.3) from 31 gallons to the 24 gallons I'd like it to show, and I copied over the sender/level values from the Jimmy into my tune.

No matter how much fuel is in the tank, my gauge reads more than full - 125% full. It's way beyond the full line. I've always just known I could get a minimum of 200 miles per tank, so I just fill up around then but I'd like a working gauge.

Any tips?

Tune attached.

It could have something to do with having values in the primary and secondary Volume tables.
I think unless you have dual tanks there should only be values in one of those.
My 2001 sierra only has values in the primary table.

I'm curious, where are you seeing secondary values? In the tune I uploaded it's zero'd out.

Huh, weird. I had 2 tables pulled up on your tune earlier that both had values. Must have pulled up the primary table twice somehow.

Have you checked the output at the level sensor and at the PCM for proper resistance?

I have not checked the resistance of the sender, that may be the thing to check next. I get a P0463 (IIRC) code saying fuel level sender voltage is too high, which makes sense that I'm seeing the code because the gauge is reading ridiculously high. What could cause that?

time for a new sending unit

car-part.com

It almost sounds like the wire is grounded out, what happens when you disconect the wire, does the gauge go down? Or maybe the reverse if it came from a plastic tank. Im thinking this is something in the wiring, not the tune. Is the sender grounded? Did it come from a plastic tank, and now its in a metal tank? They may have differant ways of being hooked up.

I would take it out and physically inspect that the level sensor is working, then go from there