[oldtamiyaphile]

When you calibrate an OBD gauge, you essentially tell it "this was my fuel consumption for my average engine speed/load last tank.

Change your average engine speed/ load (even just turn on the A/C) and your calibration goes out the window. This makes it difficult to compare driving styles.

If you take your city calibration and go on a highway run, you might be out by 10%, which is not uncommon on the cars I've got. Accelerating with fuel enrichment, likewise won't register on a system that only measures air.

I drive my Renault all city, but super consistently, last 3000 miles worth of fill ups all within 1 mpg, which gives the SGII an easy life - even then I'm lucky if SGII is within 10% (+/-5%). I know that on a long highway trip, the SG becomes little more than a guess o meter.

By comparison, a Guino that measures fuel, rather than air was able to find the BSFC sweet spot on an older vehicle that I had, a nice dip in consumption right at 80km/h, which of course became my cruise speed.