The 2005 vitz has a CVT transmission contiuons varible timing. This is the best auto transmission that can give you great miliage. This is not your normal auto transmission. Its has a laser system with the engine. It will never work well with CNG.
A guy who had the same car, said he got it convereted it to CNG after few months the tranny messed up. He had to get a anthore used one from Bilal Gung. He spent about 1, lac getting it fixed. Your best bet is to get rid of CNG. Your tranny will MESS up if you keep running your car on CNG.
I m getting about 340 km, in rs. 1500. makes it about rs. 5 per km. This is in city.