Hyundai Motor India — mileage is very low as compared to what the company has offered

Address:

Kollam, Kerala, 691601

Im a user of hyundai i20 petrol, the mileage promised was 18 but as we know it usually get lowered in city driving conditions, but other users usually get around 12kmpl, and what i am getting is around 9kmpl which is very low when you consider the mileage of a hatchback.
I have taken my vehicle to service center near my house, but they could not provide any solution for it.

Hope hyundai india will solve my problem