Did any of you find an easy, or correct way to re-set your average fuel consumption when re-fuelling?

I don't how, or why, but sometimes when I fill up and re-set my mileages, my average fuel also re-set to 7.5l/100km, but I am let to understand that you actually cannot re-set your average fuel consumption.

Any ideas if this is at all possible, and if so, how?