I have always felt that there is a quality difference in the CNG available in Islamabad vs the rest of the country. Islamabad/Rawalpindi CNG is of higher quality and results in better pick-up and mileage. The reason is probably that the gas fields in the Potowar region have very high heating value gas (over 1000 BTU/mcf and in some cases its even 1400+) whereas Sindh and South Punjab would have a tough time maintaining the standard 900 BTU/mmcfd. Also the CNG quality varies from station to station so generalised numbers can be misleading.
For the record here are the broad consumption numbers in cars I have owned/driven on Rawalpindi/Islamabad city driving with a full 55kg tank down to empty :
Toyota Corolla 2007 1.8 Auto- 120 to 130 kms.
Toyota Corolla 2003 1.6 Auto - 125 to 135 kms.
Toyota Corolla 2005 1.6 Man - 130 to 140 kms.
Mitsubishi Lancer 2007 1.3 Man - 140 to 150kms.