As stated in the subject the consumption of my car has gone worse. Previously on a Full Tank of CNG (60KG not sure but with a pressure of 230 it can store almost 10kg of gas when empty so it may be a 65KG cylinder) it was giving me atleast 150-160KM in city and around 210km on long. But for the last one month it is giving me 120-130KM in city and around 170-180in Long. I have just changed the Spark Plugs (NGK).
From where should i start to daignose the problem?