Dear Brother Shahid,
A little ago it was economical to have your car CNGed but it seems that very soon you will endup with a result which will be approximately equal to petrol. Situation in pakistan regarding all kind of fuels is verse. Since the gas is our local product so it was a little cheep. Due to law and order situation its being wasted and not being used fairly. So you will see within a year or so. The cost will be equal to petrol approximately. And that approximation will be equal to the extra weight in the trunk + frequent visits to tuner and filling stations + less power.
By the way I have 2005 honda city running on cng giving good performance but not like petrol.
For the last two weeks i could not get cng filled due to non availability in Islamabad on most of the pumps. I am running on petrol but enjoying.
Most of the honda Cities have average around 15km/l plus minus 2. So check yours and then then calculate the things.