Quote:
Originally Posted by Arragonis
I disagree, these figures are interesting but not very scientific - more runs would not make this better. The problem is the SG2.
The SG2 reads data from the car via the OBD interface - this does not include MPG or average MPG.
The SG2 calculates these by sampling values from the OBD interface - things like road speed, injector timing and dwell - which combined with engine size and the owner's adjustments sort of works out to MPG.
These sampling calls are based on time - when you do the slow runs the SG2 makes more calls and probably makes a more accurate estimate of MPG. On the faster runs fewer calls are made and therefore than result is less accurate with reality.
You can adjust this via the interface speed setting but I think that just means the SG2 should ask for data more often.
I've seen this myself in my previous Aygo. When I drove round town (usually between 0-25 MPH) I could get the SG2 to match my tank refills more or less exactly. When I drove at higher speeds (e.g. my 200+ mile trips to England this summer at 65-75 MPH) the SG2 would be up to 15% out. What was interesting was that it would be both under and over by 0-15%, not always under or over.
I think that at 0-25 the SG2 sampled the engine enough to make an accurate guess at what my MPG was. At 70 the same sample rate could miss a lot of "detail" so the calculation would get far less accurate - e.g. it could miss me coasting faster and then using DFCO and count me accelerating uphill slower more often so the average would fall.
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Am I wrong in believing the OBD-based-MPG-gauges are calculating fuel rate primarily from the MAF sensor?
OBD-II PIDs - Wikipedia, the free encyclopedia
This would be particularly a concern with acceleration testing since it will underestimate fuel-consumption if you get into WOT, unless the driver ensures that is stays closed-loop, but even then I'm suspicious MAF isn't linear across the full BHP range.
My SG2 'calibration' has drifted upwards from +11% to +30% over the course of 30,000 miles or so that I've had it. Apparently I have a sensor that is aging and yet the MPG hasn't changed noticeably.