There are two metric settings one can choose:
"Metric (litres, km, L/100km)" and
"Metric2 (litres, km, km/L)"
The Danish standard is showing fuel economy in km/L, and I have gotten used to that way. For some reason, Metric has swallowed my Metric2 chosen setting, which was working fine last time I checked fuelly. Whether I pick Metric or Metric2 as the Display Units in my settings, my fuel economy is displayed in L/100km, which doesn't say much to me.
Try deleting your Fuelly related cookies. I believe we store everything in Miles/Gallon and convert based on the setting in the cookie. So deleting the cookie and then selecting the proper setting should fix you up.
Note: We are switching to a new platform in a couple weeks so if someone reads this in the future, this might not work the same.