From my (not so accurate
) calculations I've been seeing the OBC read about 1 - 2 mpg high. Reading, say, 43.4 when I've been getting aroun 41-42. I've not worked it out for the last few tankfulls though.
I wonder if it depends on what type of driving you do as to how optimistic it is. If you have even more spare time on your hands you could track accuracy for each tank full against % spent for each tank doing motorway/town/rush hour, and maybe work it out!
