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Excuse me for jumping in here.
Scangauge is great, but it has some inherent limitations. SG is obviously relying on the OBD2 protocol. As far as I can tell, OBD2 simply does not report injector behavior (pulse width/duty cycle). Why not? Who knows. I guess the folks who designed OBD2 in the early 90s were not thinking about what a bunch of people are thinking about now: accurate, realtime reporting of fuel consumption. I think monitoring injector activity is probably the best way to do that, but OBD2 doesn't get us there.
Since the injector information is not available, SG calculates fuel flow indirectly, by relying on something else that OBD2 does report: air flow. I guess that method works well enough on most vehicles, but it seems to have problems (at least for some people) on two types of vehicles: diesels, and vehicles with lean burn (Honda VX, HX and Insight).
Lean burn is a challenge for SG because it's relying on the assumption that mixture is usually close to stoich. OBD2 does provide O2 sensor data, so I think theoretically SG could make corrections, using this data, but I think it doesn't do that. Anyway, that's still indirect and prone to error, compared with watching the injectors directly. Trouble is, OBD2 doesn't allow that.
I think something similar might be happening with diesels. I have a feeling that SG will have an accuracy problem with any vehicle that is not running mostly at stoich, for whatever reason.
A digital DMM with a dwell feature ($33) can monitor the injectors directly. That might be a good way to go if you want to watch instantaneous fuel consumption in a more direct manner.
Anyway, maybe this answers some of the questions that were raised. Someone please speak up and correct me if I'm missing something.
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