W.S. Generally I think garmiyo'on mein aesa ho jata hai. I own Passo X, 2013 model and meri b fuel efficieny 20.3 kmpl within city (during winters and with 5w-30) sey 14.5 kmpl with AC within City p aa gye hui hai 
(but iss mein kuj meri galti b hai, I poured 5w-40 during last oil change). With light foot and 5w-20 or 5w-30 oil, it should easily give 15 kmpl + with AC within city, but the condition is; your car should be well-maintained!
I think you need to check the following:
Throttle body service,
Cleaning of plugs,
Check for clogging of Catalytic Converter,
AC gas zyada nai honi chaheye, wo b dbaoo dalti hai,
Use good quality fuel from a reputable brand (and I do not necessarily mean High Octane! 
),
Check / replace Air filter,
I do not know which oil you are using but perfect oil for your car would be API SN, 100% genuine / original Fully Synthetic Oil with Grade / Viscosity 5w-20 or 0w-20 (these both are perfect grades for Vitz / Passo) up to 5w-30 or 0w-30 Max.! (inn sey uper k grades engine ko heavy karein gey!)
Maintain Tire pressure as per OEM recommendations and check it every week at cold,
Tire sizes should be near to stock,
In the last, agar aapnay kafi arsay sey wheel balancing / alignment nai karwai, aik baar check karwa lein, although a small thing, but it all does matter. Do revolve tires in their defined pattern after they do 10,000 kms.
I hope, it helps. Thank you. 
