As used in: Only a few new cars, such as the Proton ecologic range, but there are many used cars converted to run on the fuel.
What is it? Modified cars can run on liquefied petroleum gas (LPG) or compressed natural gas (CNG).
Where can I buy it? At about 1400 fuel stations across the UK.
Pros: The cost of gas is low and it has lower emissions of carbon dioxide and hydrocarbons than petrol and fewer particulates than diesel.
Cons: To date, there is relatively limited supply in the UK. Gas must be stored in
high-pressure tanks and has poorer mpg than conventional fuels. Government grants for gas-run cars are no longer available.
Expert view: Gas has not taken off as quickly as the Government would like, but it's still a viable alternative.
Outlook: Needs further commitment from government and car makers for it to catch on. Like petrol and diesel, however, LPG and CNG are finite resources.