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You may have something there. I have never checked the odometer against the GPS. I don't really know how I could do that. I do know that the speedometer reads about 10% slow verses the Sat Nav. 30 mph on the speedometer is 27 mph on the Sat Nav (and roadside safety speed indicators). That would mean that if I drove at a speedometer reading of 30 mph for 10 hours I would have actually only done 270 miles where the odometer would say 300. That is even worse than the figures I actually measured which would equate to 29 mph at an indicated 30. Mind you, it doesn't seem to be a uniform 10% at all speeds so that may help explain it.
I'll keep logging my calculated mpg and treat the ScanGauge as an indication. After all, that is what I have done for the past 50 years! I'll just be aware of the difference.
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