The legal position is that Cars or motor vehicles are permitted to run on CNG, which have stronger - thicker cylinders to take the rough on the road, unlike LPG cylinders which are subsidized for domestic use, & designed for stationary applications at home or industry. It is popularly used by taxis & people travelling a lot as it costs half of petrol & gives high speed & less pollution. However, u may be hauled by the police for unauthorized use.
People cover up the cylinder area with dark film so it is not noticeable to viewers or to police.
The cost of conversion is about Rs.10, 000/- for LPG, 50%more for CNG.
LPG cylinders are easily available. But marketing & availability of CNG is not widespread yet. It has started mainly in metros only.