Regarding the reliability of the OBC:
On my car, for instance, there is a KVA option in the hidden dashboard menu. The KVA has to do with consumption variance and how the car reports fuel economy. BMW's instructions for calculating this value is to fill the car, drive it, and then compare the calculated economy (A) vs. the OBC (B) economy. Then do the following calculation: (A/B)x1000=X. (X) would then be the actual KVA.
The 7-er was on a KVA of 1000, and since I am meticulous with fuel recording, I noticed often large differences between my calculated economy vs the OBC. I then changed the KVA to (X) based on my calculation, and my OBC vs calculated economy is now within 0.5l/100km of each other where in the past the difference could be as high as 7l/100km.