The OBC could be adjusted on my Z3. I am sure it is the same way on MINI's. I had the OBC value within 0.3 mpg of my manual calculations. Search bimmerforums.com.

 

I've been driving pretty very conservatively on this tank of gas trying to get max mileage. 50/50 mix of highway and city OBC shows 33.8mpg. I'm about due for a fillup so I'll be calculating actual probably tomorrow. Last tank obc showed 29.5 and calculated 28.9 (lots of WOT and spirited driving on that tank).