Tata Motors — Nano xta performance

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411038

I have purchased tata nano xta model in the month of june 2015 from pandit automobiles pimpri branch. From the day one i am not satisfied about the average of the vehicle. It is told by the dealer and very much mention in the advertisement that the millage of the new nano car is 21.9/ km. So i am expecting 18-19 kilometers per liter. I am getting only 11 to 13.5 km / liter. I have already complaining this to pandit automobiles sahakar nagars workshop but they could not able to find out the solution.
I am expecting help in this regards. Otherwise i have compel to sold the vehicle and go for other good brand.
Thanks and regards
Sunil k