mathew -
After I made this post :
http://ecomodder.com/forum/187883-post20.html
I came to the conclusion that these are the most important parameters for deriving MPG :
MAP
IAT
RPM
MPH
However, you always have MPH and you usually have RPM "for free" on your instrument cluster. For observation in the ScanGauge, I would look at these instant parameters :
MPG
MAP
LOD
IGN
If I had only one open slot like you are stating, I would pick MAP first and LOD second.
CarloSW2