@VITZ: My friend who owns a 2004 vitz told me that he gets around 90-100+ km/full cylinder with pressure 200. If the pressure is more than 200 then you are more likely to get more mileage on CNG. He also said that the average he gets on CNG is on the busy streets of Karachi that means traffic jam and all plus he never runs it without AC. I believe 90-100+ is good for an auto transmission car. My brother's Suzuki alto does 100-115 depending on the car's tuning.
I would like to add something to this whole CNG vs Petrol thing. Basically, I have also been trying to calculate stuff. I believe that if you don't plan to running your car more often then I guess running Vitz on Petrol should not prove to be that expensive. If you take your car out to your work only and your route is not heavily jammed every now and then, then I guess Petrol should be the first and only choice.
Now, if you are planning to use your car more frequently, if you visit your friends, relatives etc more often then you might wana consider CNG i.e. unless of course, you have got $$$ in your bank and don't really care about petrol expenses.
Infact someone told me that Vitz costs 4 Rs/Km on petrol and 3 Rs/Km on CNG. If this is true then there is not much difference in fuel average whether to use petrol or CNG. I don't understand then why people convert it to CNG.
See there is a difference between the cost of CNG and Petrol. You get 1 liter petrol for Rs 54 and you get 1kg CNG for 33. The difference is either costing or saving you Rs 21. So, that means that even though your Vitz is producing almost the same results as it is on Petrol but it's costing you less. Also, if you are saving 1 rupee on every kilometer that you drive then know that you'll be saving 100 rupess every 100 kilometer.
Basically, it depends on your useage, the area/city where you live and your driving techniques (I am yet to learn more about driving techniques).
I will be getting my Vitz in 5 days inshAllah and I am planning to run it on Petrol for a month or so. I am gona see what results I am getting and how much am I paying for the Petrol. If it costs me an arm and a leg (and I am sure that it will), I'll defenitely go for CNG BUT, I will make sure that I drive it on Petrol for a few kilometers every other day if not every day.
Just my two cents