[dcb]

OBD mpg gauges usually makes an assumption that the air fuel is close to 14.7 to 1, which is reasonably accurate for a modern fuel injected/o2 sensor feedback gasoline car.

On diesels the air is not throttled, thus the air fuel ratio is all over the place, usually very lean, so you cannot expect consistent results.

I would expect that subtracting .2 l/100km from the OBC in your head would be the way to go for accuracy. the on board computer should have a very good idea how much fuel it is injecting, or at least be off by a consistent amount.