CNG on new honda city should not be a problem coz its IVtec engine not IDSi, also there is no CVT in it. Honda people say it has same kind of transmission as of Civic Prosmatic. So in my opinion there shouldn't be any problem on new city running on CNG. But there are some other issues like Warranty etc. I own Auto City 09 and not thinking of Putting CNG coz of few reasons like, I dont want warranty to void, secondly i need full boot space to put my wheelchair in it, and also my usage is not very excessive so that i think i can run it on petrol atleast for the time being.
Now coming to the average of new city, its not yet calculated as cars are still young and new cars start to show their best average after about 10k kms, my car have still done only 3k and its average is around 13kms with AC on Highways and around 10kms again with AC in the city but m not satisfied with it, hopefully average increases after some more kms.