kindly elaborate your car's make and model... is it civic or corolla?
Anyways, you want to calculate the mileage you are getting from cng... Just fill the cng cylinder from empty, note down odo reading, the kgs of gas filled and amount of rupees paid, then drive till your cylinder gets empty again and then get the kms travelled. rest is simple maths...
You mean the petrol average which is shown on the gauge cluster? That shows instant mileage and calculates average mileage on the end on the trip... Here again an average is taken for the whole trip.
In cng your trip duration is the distance traveled in whole cng tank consumed.
The variation in every trip can be due to many reasons like cng quality, pressure, car tuning, tire pressure, driving style, terrain, ac, occupants and loading, ambient temperature and pressure, etc etc