Swift dzire cng car petrol mileage redcues in petrol mode

Hi,

I am having swift dzire petrol 2016 model .
In Dec 2018 i had installed Sequential CNG kit from authorized CNG fitment center in Delhi.
So i just want to know when we run CNG fitted car on petrol mode so how much petrol mileage it should give or it will give the mileage as earlier it was giving when it was a actual petrol car.

Or it will reduce the petrol mileage after CNG conversion.