Compared to my scangauge my OBC actually reads conservatively. The scangauge readouts seem to be about 2% high while the OBC is closer to 10% low on my car.
Do you mean if the OBC reads 30 MPG, when you do the math you really get 27 MPG? That's what our 08 Clubman does. Mark
The other way around. The OBC would say 28.xmpg over the average of the tank and mathematically it worked out out to just over 31mpg at the pump. With instantaneous readings the scangauge often reads about 4mpg than the OBCs instant readout. I guess with so many cars being optimistic at least one car had to roll off the line with a pessimistic OBC to make the average readings amongst all MINIs more correct. Oh, and the speedometer isn't so far off either (at least at the vehicle speed sensor) with my winter tires ~30mph on the scangauge shows as 30mph on an external radar and with the summer setup 30mph shows as around 29mph.