By Chance i also happened to visit the suzuki dealer recently for the same issue of low milage of my wagon R. it has just touched 6000 KMs & the experience was similar as narrated by above members. Tech. informed will do engine tune up & OBD Scanning.
so he clean the Engine Plugs, Clean the Air Filter & used OBD Scanner. (in which no fault code detected). i told to make ECU for better economy as i used to read from other pakwheels members ,but tech. told its a myth in customer minds & there is no such thing or Setting in OBD Scanner for better economy.
Tech told after filling full tank, run the car for 50 Kms & then again refill to full, & note the average per kilimeter .hoped average will improve. Thats all.
Now all i can do is to observe the Km/L average in the coming days, if it improves, i will inform again. currently wagon r vxl is giving 10.5KM/L with AC in City & 12.5 KM/L with AC on motor ways.