Tata Motors — tata nexon xm petrol mileage complaint

Address:

House no 782 first floor milk colony dhanas, Sector 14 west Chandigarh

Hello,
I have bought tata nexon xm model petrol in november 2018, i only have issue with the mileage as company quote more than 18 km per liter but from the beginning my car is only giving mileage of 11.5 km per liter in city and on highway it goes upto 14.5 km per liter not more than that.
I raised this query at service center as well but no response was given.
My car has already done 26000 km and i am really upset with its mileage.
Kindly do the needful as soon as possible.
Regards
Hardeep singh