Not sure if it is a bug or not, but when exiting the game and start again. the average fuel consumption resets to 0L/100KM. I don't know if IRL Scania this happens or not, but ingame, if we do a long trip and for some reason we have to exit the game, when finishing the delivery we can't know for sure what the trip average was.
Tested in 1.6.1 vanilla game, no dlc, with new created profile and in quick job.
Two pictures for enlightenment:
Before exiting:
After exiting and start game:
