There are now three major car sharing fleets operating across four capital cities in Australia along with two peer to peer networks allowing people to rent their personal vehicles to others.
A car that can be replaced by a car sharing service will most likely be getting relatively little use each year. The embodied emissions (those created in its manufacture) will likely far out weigh those produced by its use. Further more once the annual runnings costs of registration, insurance, maintenance and the owners time are tallied up, the per use or per km cost will often be significantly higher than the equivalent car sharing service.
