<Tr> <Td_colspan="6"> American Council for an Energy - Efficient Economy Greenest Vehicles of the Year </Td> </Tr> <Tr> <Td_colspan="6"> Greenest Vehicles of 2012 (Top 5) </Td> </Tr> <Tr> <Td> Mitsubishi i - MiEV </Td> <Td> 2012 </Td> <Td> Electric car </Td> <Td> 112 mpg - e </Td> <Td> 3.8 mile / Kwh </Td> <Td> 2.9 mile / Kwh </Td> </Tr> <Tr> <Td> Honda Civic GX </Td> <Td> 2012 </Td> <Td> Natural gas </Td> <Td> - </Td> <Td> 27mpg - e </Td> <Td> 38 mpg - e </Td> </Tr>

What is the least polluting fuel for vehicles