Thank you all for the wonderful responses, and I can see how this challenge might be more than I'm prepared for, as a total beginner, but I'm not ready to give up just yet.
Rockrockmcrock, No worries of thread jacking, I'm here to learn so more info in any way is useful.
To answer your question, this vehicles been converted over to Natural Gas. Its tank reads in PSI from 0-3600. I have a digital gauge accurately reading pressure, so I know my fuel level. The state of California requires the OEM gauge to function appropriately as well.
To all the others, Sadly, this went over my head quickly. Anyone wanna recommend a good place to start learning small electronics and math? haha
I understand the fuel curve of the OEM gauge + sender weren't a straight line but curved and therefor would take a more advanced equation to solve for every voltage to resistance value of the new sender. I'm not looking for something that accurate, and I don't mind if its off a ways, as long as full is full and empty is empty. I was thinking Digipot because it sounded like the simplest, smallest form-factor solution to my issue. My overall goal here is to get it working as simply as possible. As an alternative thought, could I pick up a variety of resistors; a few 1 ohm, a pair of 5ohm, a 10, a 25, a 50, a 100 and a 200, stick them all in a board in a matrix and use a controller + programming to step it up 1 ohm at a time? I feel like building a simple program that reads voltage and routes a circuit off of it would be relatively easy. That could potentially result in my OEM gauge going through multiple steps as defined by the program at various voltage inputs. This would be more than accurate enough for my needs.
This idea came up after looking at the OEM sender again yesterday afternoon. It is a two wire sender, with a basic float on an arm that travels across multiple steps from empty to full - its not a constant resistance change. I didn't measure each step, but id say there was about 50.
My goal here is to replicate the OEM set up as close as possible, so I dont have to modify anything, just 'replace' the component I removed.
What potential problems could I see from this type of setup? How hard would it be to set up a board that let me program a circuit to run through multiple steps the way I'm thinking?
The reason I'm trying to keep it so simple- The OEM sending unit was extremely simple. like I said, about 50 different steps of resistance as the fuel level dropped. The OEM computer/gauge side of things is way more complex than I fully understand. Its a 2 wire set up, I can pull a wiring diagram if anyone wants to see. The computer sends a PWM signal and listens for results. It also seems to send a check signal at start up, a slightly larger square wave, and it sets a gauge fault if the resistance is out of range (open/to high/to low). All I had to do to get around that is shove a 120ohm resistor in place of my OEM sender. That gets it through start up, but after about 100 miles of driving with no resistance change it sets a fault for stuck gauge. That brings me to this project.