A large-scale study of 6,281 carsharers by University of California Transportation Center shows that over 80 percent of those entering a car sharing program dropped their personal vehicles when they joined. For instances where participating in car sharing didn't completely eliminate cars in a family, it often stood in for owning a second car. Not only did that mean fewer cars overall, it also resulted in fewer miles driven and the cars in the car-sharing program had, on average, considerably higher fuel economy than the vehicles they replaced.
Fewer vehicles. Fewer miles. Higher mpg. Put that all together, and you've got a good formula for reducing both oil use and air pollution. Car sharing companies like Car2Go and Zipcar are now looking at ways they can expand outside of urban cores.