I tried this last week, and hit 160mpg lol only for a minute on MFD 1
Took car for about 7-ish mile drive; turned it off, left it for a while 30min or so. Started it up again and drove off, MFD was still displaying the 40mpg from first journey. So i reset it on No 1 setting to zero, carried on with the journey and the mpg was
If that makes any sense give it a go.