The much-maligned diesel has a new moniker that is more impressive than its dirty fuel tag. In the Indian automobile market, diesel is king, and it is now the cheapest automobile fuel, edging out the green fuel CNG. These days, diesel works out cheaper than CNG. For instance, if you calculate the operating cost of CNG in Maruti’s popular hatchback Swift or the Hyundai i20, it emerges that the cost is higher for CNG than diesel — the average per/ km running cost of these cars is around . 2.10 on CNG and . 1.77 on diesel.