Check before buying TATA Vehicle with existing Customers about Milleage

I have purchased in the Year 2017 FEB top end XZ (Petrol) with high expectation and from first week itself im complaining about Milleage and these people said me after first and second Service milleage will increase but it has not happened after third Servicing also and im really fedup with performance the Tata Car. in city traffic i get hardly 10 to 12 kms and in high way 17 to 18kms. Coming to Riding Experience not good or Bad., Interior wise it is good and exterior ok type, still now i have drived 21000 KMS only in 3year so only lubracant charges only, If milleages problem is solved it can be good vehicle as per pricing pricing