CNG is a cleaner alternative to other automobile fuels such as gasoline (petrol) and diesel. By the end of 2012 there were 17.25 million natural gas vehicles worldwide, led by Iran (3.3 million),
Pakistan (3.1 million), Argentina (2.18 million), Brazil (1.73 million), India (1.5 million), and China (1.5 million).The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines. Gasoline/petrol vehicles converted to run on natural gassuffer because of the low compression ratio of their engines, resulting in a cropping of delivered power
while running on natural gas (10%–15%). CNG-specific engines, however, use a higher compression ratio due to this fuel's higher octane number of 120–130.