Ken, you mentioned in that thread about the non-linearity of the fuel gauge response, even in an OEM application. I've been half-baking ideas to try and fix that on my Dodge for over 3 years...
http://www.pirate4x4.com/forum/showthread.php?t=638897&highlight=linear+fuel+gauge
I think my original idea had to do with recording the resistance at each gallon mark to build a calibration curve for the stock sender, then use that to get a straight % of fuel remaining from the sender reading, and convert it to the appropriate resistance for whatever gauge I was using. Basically obtaining both calibration curves by experimentation and creating one function to go between the two.
Just what I need, more project resurrections...