Visit the Alternative Fuels Data Center and read Natural Gas Vehicles [1] (subpages, too, if you like, but not required)
The following sections, giving an expanded view of Natural Gas Vehicles (NGVs), are excerpts from HowStuffWorks [2]
Advantages
The biggest advantage of NGVs is that they reduce environmentally harmful emissions. Natural-gas vehicles can achieve up to a 93 percent reduction in carbon monoxide emissions, 33 percent reduction in emissions of various oxides of nitrogen and a 50 percent reduction in reactive hydrocarbons when compared to gasoline vehicles. NGVs also rate higher in particulate matter 10 (PM10) emissions. PM10 particles transport and deposit toxic materials through the air. NGVs that operate in diesel applications can reduce PM10 emissions by a factor of 10.
Natural-gas vehicles also offer these benefits:
Disadvantages
One of the biggest complaints about NGVs is that they aren't as roomy as gasoline cars. This is because NGVs have to give up precious cargo and trunk space to accommodate the fuel storage cylinders. Not only that, these cylinders can be expensive to design and build – a contributing factor to the higher overall costs of a natural-gas vehicle compared to a gasoline-powered car.
Another drawback is the limited driving range of NGVs, which is typically about half that of a gasoline-powered vehicle. For example, Honda's natural gas Civic, the Civic GX, can go up to 220 miles without refueling. A typical gasoline-powered Civic can go approximately 350 miles without refueling. If a dedicated NGV ran out of fuel on the road, it would have to be towed to the owner's home or to a local natural gas refueling station, which might be harder to find than a "regular" gas station.
Finally, it should be noted that natural gas, like gasoline, is a fossil fuel and cannot be considered a renewable resource. While natural gas reserves in the United States are still considerable, they are not inexhaustible. Some predict that there are enough natural gas reserves remaining to last another 67.1 years, assuming that the 2003 level of production continues.
NGVs can be built from scratch to include the design enhancements described above. A brand-new natural-gas vehicle costs $4,000 to $8,000 more than a comparable gasoline vehicle. It's also possible to modify conventional gasoline vehicles to run on natural gas. This, too, can be expensive, with the modifications typically costing $3,000 to $5,000. Vehicles that run solely on natural gas are known as "dedicated" NGVs. Vehicles that can operate on both natural gas and gasoline are known as "bi-fuel" vehicles. In bi-fuel vehicles, the driver can safely switch from one fuel to another while driving.
Links
[1] http://www.afdc.energy.gov/vehicles/natural_gas.html
[2] http://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/ngv4.htm