Petrol Avg depends on proper wiring and sensors. I get around 13.5 to 14 KMPL with 1800CC 16 Valve toyota engine your should do 14+ with light foot local.
CNG I get 200 with 55KG Cyl. Now this also depends on CNG settings but it will do around 170KM.
Which engine you Got? 4EFE? Can you take picture of the engine bay and post.