After going through the forums, I managed the "DIY" to resolve the issue of rpm increase. Following procedure was followed and issue was removed.
1- Disconnect the +ve battery terminal
2- Disconnect the MAF Sensor
3- Take off the MAF Sensor and Put it back
4- Connect back the MAF Sensor's power cord and +ve battery terminal
5- Start the car and issue was gone 
Maybe ECU reset did the job and I did extra work of removal and putting back the MAF Sensor.
Anyways, My car is doing 17km/kg of CNG. is it reasonable or it have to do more? Its average of in-city and highway drive without AC.
One thing to be shed by experts, what will be the average milage on 1 liter pertol in-city and highway with and without AC?
One Issue to be added, after crossing through road bumps and speed breakers, a strange sound comes from rear as if the car is hit to the speed breaker? it may be the shocks but they seems normal on straight road with minor bumps. please suggest what to do?