My experience says; by installing CNG on your car, your car won't give the same mileage on patrol as it used to do before installing CNG in it.
The reason is lots of connection and settings got disturbed. If your car is EFI then off course advancer and emulator gets added in the connections. Different connections through sensors also. i.e your car has alteration now.
So It won't give the same feel on patrol in terms of pickup, drive and consumption too.
Thats only my observations so one can neglect it off course.