Wasalam bro, as far as my experience is concerned, if you want to keep your Civic on CNG then it is better that you'll buy manual transmission Civic rather than with the automatic transmission as it's consumption is really better on Manual transmission, though CNG is not recommended on Civic. 
Anyways, as far as my Civic is concerned, its giving me like 150km on full tank local and around 200km on long route, so I'm pretty much satisfied with its consumption on CNG and it's petrol consumption is also better like 15 km/l. 