Geonerd, can you elaborate on what data to collect from the ScanGauge? Just instant MPG, or something more?
I haven't thought it through completely, but something like this has been bouncing around in my head for a while...
Find several long, even hills (poor man's dyno) or various slope (increasing load). Drive up them in different gears/RPM, at different speeds, logging instantaneous fuel consumption and throttle position. I don't even have a SG (no place to plug it in!), but I think they can record T.P., as well as the raw fuel consumption rate, right?
If instant consumption values aren't available, MPG would work as well, so long as you factor out the speed, and wind up with gal/hr, ml/min, etc.
(Is there a SG version that can log several minutes' worth of data? That would kick butt!)
Weigh car. Survey hills. Calculate how much power is required to lift the car up the incline at any given speed. Perform coast-down tests (on level ground!) to determine the power required to overcome rolling resistance and aero at any given speed. Add the two to get (theoretically!) the HP/KW needed/produced to maintain speed up the given hill, at the given speed, and divide into recorded fuel consumption. If all has gone per plan, you now have a number representing power produced per fuel consumption rate. "To make X HP/KW, the engine requires N gallon/grams/liters per minute/hour."
Whatever the units, plotting the resultant values on a grid, located at the appropriate TP and RPM intersection, should make a crude BSFC map.
OK, what did I miss?! (I musta missed something! )