Dear Daniyal
Maybe there is some problem with your car. I agree that the average is neither 20 nor 18 in Karachi, but blv me even after using Mehran with heavy foot with AC on I used to get an average of 11-12 kmpl and agar insano ki tarah chalao to 13-14. And this was for Mehran VXR 2013 (AZD-xxx) which was my company car in Pakistan. (Returned the car with 16K on ODO)
The other one I have is also 2013 (AYX-xxx) its VX with after market AC. Did not have time to check its consumption after AC installation..... but without it I was getting average in between 15-17 kmpl easily with light foot. (This car is currently 15k driven)