I also have a Mehran at home, it has only done 8k kms, average in Karachi city is 70 to 75 kms / 40 KG cylinder.
So no worries, just fine tune the CNG kit and you ll get better average for sure, in Karachi city's traffic you cannot get 100 kms, in Islamabad you can. On highway your cars average will exactly be double, this Mehran we have does 150 kms on Super Highway