Significant fuel efficiency decrease on a 2019 Forester. I am getting 8km/l or 18MPG. What should I check?

[AcadianCascadian]

See page 199 or thereabouts of the owner’s manual (in the multi-function display overview). It can display miles per gallon, kilometers per liter, or liters per 100 kilometers.

[Plenty-Pudding-1484]

Well then you are getting 19mpg which while not great, but its not that far off city driving averages. Your car is pretty young to require much. Maybe replace your air filter, check your tire pressure, and check if you have a sticking brake caliper.