[fixit-p]

Originally Posted by VetteVet

FIXIT-P isn't 90 ohms what we are looking for to convert to the camaro gauge. The total resistance at full should be 90 ohms. I believe Doc said to use a 100 ohm pot. This is starting to make my head hurt LOL

Vette, I think the OP has a 0-30 ohm gauge that he wants to work with a 0-90 ohm sending unit.... that makes 2 of us with a head ache .

Originally Posted by k1rodeoboater

From DocVette....he actually describes how to do BOTH 30 to 90 and 90 to 30 conversions in that thread. LOL no wonder why this is causing some of the confusion and headache.

Yeah it does seem like there is a contradiction in that thread.
No matter what you use it should go in parallel and not series. the pot may start out as adjustable but once it is set and wax or other sealant is applied to the stem it's just a fixed resistor at that point.
It's true that you will only be accurate at full and empty because as the fuel level goes down so does R1 which means the value of R2 would need to change to give you a proportionate resistance which we know cant happen because R2 is fixed at ~ 47 ohms. Example at 3/4 of a tank on a 0-90 ohm sending unit the resistance is 67.5 ohm and on a 0-30 ohm sending unit it's 22.5 to get a correct reading on the gauge it would take about a 34 ohm resistor in parallel with the 67.5 to equal 22.5.
Below is a chart of the actual fuel level compared to gauge reading with this mod. the accuracy is based on theory and subject to my math but it will give you an idea of whats happening.