I suggest not to go for CNG, as its almost same as Petrol if you calcualte for some period of time.
- CNG Equipment Cost Recovery
- Faster Engine Deteroriation
- Frequent Tune Ups
- Frequent Filter and Oil Change as compared to Petrol
- Over heating as compared to Petrol runned Engine
- 40-50% Trunk Space Gone
- More Weight at Rear Wheels
And many other things as if by bad luck you got bad fitter then multiply it. If you can afford for Petrol then keep away from CNG otherwise its compromise and short time saving.