Hey congrats on your purchase,
I have the same car in Black and have been going through the same tires dilemma as you.
Firstly I would suggest you definately do NOT install CNG on the vehicle,
it's a very economical car and especially after 3000 to 5000 km, i.e. once the engine has been run in it gets more economical than when its first out of the showroom,
mine's done around 3500 so far, I'm using a 50/50 high octane to supreme mix on the car and the efficiency is awesome and the engine seems very happy too,
keep the revs low i.e. under 3000 rpm and its pretty economical,
now for the tires...
the installed tire size is 175/65R15, and a lot of people have been bitching out Honda Atlas for saving money and installing tires which are too thin, agreed they are certainly too thin to look at,
So, next I did some international benchmarking, the similar (but not the same) model in the UK is the Honda Insight and guess what the factory spec tire size is on that...
175/65R15 reference: Honda Insight | Book Test Drive | Specification, Finance & Information
So then I thought I would travel some more and went to the US, guess the tire size there.....
175/65R15 reference: 2010 Honda Insight Hybrid - Specifications - Official Honda Web Site
However, despite being similar, these cars were not a City, so I went to India, and found...
175/65R15 reference: :: Honda City ::
Now one must bear in mind that these cars have been designed with economy in mind, not performance or sportiness, so the logical sequence for me to follow was to check out the people who make a Honda sporty, Welcome to the world of MUGEN... and guess what tire size I found there...
Bridgestone Potenza S-03 (195/50R15)
It is however relevant to bear in mind that the Mugen city is also lowered, and empty looking wheel arches are hideous, the stock tire size (175/65R15) radius is 12 inches, whereas the Mugen size (195/50R15) is 11.3 inches in radius, and lesser radius = lesser comfort,
Assuming that you are not lowering your vehicle, keeping aesthetics, comfort and factory spec in mind I would recommend 195/60R15, which is 12.1 inches, an error of 0.1 inches which can be ignored...