Hyundai Motor India — Hyundai i20 active s variant not giving average as claimed

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I purchase i20 active s (diesel) in july 2017 from kamal hyundai kharghar - navi mumbai and i was expected to get at least 18-20km/ltr average but after couple times i tested but it gives me hardly 15 and getting lower day by day. i have requested o service center as well before by submitting it for for first free servicing but i guess they have ignored the issue and i am still facing this issue.

Could you please suggest and give proper resolution for this concern?