agreed with Naxun,
cultus efi remains on petrol for first 90 seconds, after that when you push accelerator to go rpm above 2000, it is automatically converted to cng.
Its better for engine for long life, otherwise we don't run car on petrol which reduces its life, so its advantage of cultus efi, engine running remain smooth.
3000-3500/ month is huge.
it should be less than 1000/ month.
i think you mostly ride your car in city?