Marty,
I also own a 2000 C5 and am very active in ecomodding activities on my car.
I did some very successful underbody mods and other things ranging from Engine software changes like lean-cruise to solar cells to engine and trans heaters for cold starts.
http://ecomodder.com/forum/showthrea...ette-9108.html
I use the Vette as my daily driver with current hwy mpgs between 28-31 (A4 with H/C LS1). My city average is 20 mpg.
Regarding the Scangauge, I think we can get away without one because we can display instantaneous MPG in our instrument cluster, which is very helpful i driving economically.
My next steps are an underdrive pulley for the cankshaft and maybe an electric waterpump. I've also looked into converting to an electric powersteering pump. There are many options out there that might make sense but they all involve some work and $$.
Alex