The Mahindra Supro CNG Duo comes with a bi fuel system which can run on both CNG and petrol. The marketing message is that this flexibility and this peace of mind is offered to drivers, particularly in areas where CNG infrastructure may not be as robust as in other areas. But, some question if this dual fuel capability is actually useful in the real world or more of a marketing gimmick. The small petrol tank comes in for criticism from some, who say it doesn't offer enough range to be useful, but others welcome the backup option. It also raises questions about the complexity of dealing with two fuel systems and how that compounds long term reliability. Another is the added cost of the bi fuel system. In order for fleet operators and individual owners to make an informed purchase decision, it is important to understand the real world benefits and drawbacks of this feature.