Swift auto can and should run on CNG as it has got gasoline engine. I have seen many automatic cars running on CNG. Got Honda city automatic in past which was on CNG and performance and mileage was quite good. The power will be lower when driven on CNG as it is a gas. But you can drive it without much hassle. Regarding fuel average, that is a common problem with cars. Over time, engine wears out plus fuel system starts getting weak and acts abnormal. There is another aspect which is ignored by people if they are getting poor mileage. Frankly speaking, oil companies are thugs and they do add some unknown additives in gasoline which leads to increased fuel consumption and lower mileage. These additives also clog the injectors, fuel pump, fuel filter etc resulting in an increased maintenance cost.