Bought my second car Scorpio N

I have purchased diesel variant Z2. My driving split would be 75 highways -25 city. Maybe 3000 kms a month. I never off-road apart from potholes on the roads. I just completed 1000KM in the first 15 days. My mileage is very low 8-9 in city and Highway 10-11 maximum. I heard it gives around 18 on highway diesel if you drive decently. How can I make it like that currently it’s too low.