The economist has a good piece on auto fuel.
LNG (primarily Methane converted to liquid form) is used by a lot of public transport vehicles in Asian countries.
Natural gas can be considered as the most environmentally friendly of the fossil fuels, because it has the lowest CO2 emissions per unit of energy and because it is suitable for use in high efficiency combined cycle
power stations. Because of the energy required to liquefy and to
transport it, the environmental performance of LNG is inferior to that
of natural gas,
although in most cases LNG is still superior to alternatives such as
fuel oil or coal. This is particularly so in the case where the source
gas would otherwise be flared.
CNG is propane fuel. It is considered by some to be a more environmentally “clean” alternative to those fuels, although it produces greenhouse gases, and it is much safer than other motor fuels in the event of a fuel spill: natural gas is lighter than air, so it disperses quickly when leaked or spilled.
Compare those to the hybrid Prius.
CNG car gets about the same fuel economy as its petrol equivalent, but
emits 80% less smog-forming nitrogen oxides. The Environment Protection
Agency rates the Honda Civic GX—the only production car sold in America
that runs on CNG—even cleaner than the Toyota Prius.
image credit: Volvo