Don't install CNG as the car's engine will die,
What's the mileage of your car?
To improve fuel efficiency try doing the following.
- Change Oil & Oil Filter every 3,500 kms instead of 5,000/-
- Change the Spark Plugs (Don't clean them & put them back)
- Change the Air Filter with the original one not guard filter (the thickness of filter paper does matter)
- Get the throttle body serviced.
Let me know after doing the above 4 what average are you getting, I get 13-14 ki average on Passo 2007