Well, in theory, your car will gets its best MPG if there is forward momentum while your engine is pulling max vacuum, right?
One way to monitor this would be to " T " into a manifold vacuum source and run a vac/boost gauge to your dash and monitor that. I have done this on both my Saturn and my Z28, to prove to a friend that cruise control wasted fuel. I just routed the vacuum line up out of the hood, turned the wipers on and pulled the fuse while in the up position and zip-tied the gauge to the wiper arm on the windshield and went out for a few test drives.... worked great. Cruising at steady state on flat surface I was at 19" vac, turned cruise on at same speed and dropped to 12-13" vac. More fuel for same speed = wasted fuel.
Would another sufficient way be to use the MAP readout on the SGII? Manifold Absolute Pressure, right? Watch it, manipulate the throttle to keep MAP readout as low as possible, maximize FE.
Just something I was thinking about today while reading up on turbo talk.
Maximum vacuum is a waste. Minimum vacuum means no energy wasted pumping a vacuum. Folks have mentioned cars that came with a FE gauge that was just a vacuum gauge; this would work for people who don't even vaguely try to drive efficiently, by convincing them to accelerate easier and drive slower. With an automatic it would probably work ok too, because you would keep it in higher gears when obeying the gauge. Even better is to be able to plan ahead and drive according to the road rather than just driving according to instant feedback.
If you've already got a ScanGauge II, watching instant MPG or cost-per-mile does a good job telling you how effective your right foot is...certainly better than TPS or MAP.