There is an edit vehicle button next to the vehicel type in the profile. You can change the units there.
New car profiles default to the units set in the user profile, not previous cars.
Not in my instance.
Display units are the same but the input units are gallons and not litres like my other car.
Want UK/mpg but litre inputs when adding fuel up.
Works on other car. All I did was add a vehicle - no setting changed.