Maruti Suzuki India — Mileage issue

Address:

304A Mercury, Patels Planet, behind father agnel school Ambernath west 421501

Dear team,
In last November I had purchased a Swift ZXI AGS, (Petrol) at the time of booking the company has suggest and assured the Car mileage will be more than 20 kmpl but I have facing mileage issue in my car. I just get mileage around 12 kmpl.
Please help me to resolve the issue.
MY CAR Chassis number is MBHCZCB3SNKB16199
Regn. No. MHO5FB1804
Engine No. K12NP1320024
RC book number :- MH24506602

Swapnil Pawskar