Energy use in the transportation sector (i.e., gasoline, diesel, or liquefied petroleum gas), has caused urban air quality to deteriorate and is responsible for a significant amount of greenhouse gas emissions. First, ensure your car is operating as efficiently as possible. For the future consider investing in a hybrid or even better, an electric car. This blog has a great summary of the efficiency of different models.
If you are considering buying a car, double check if you can use alternatives such as public transport, cycling or renting a GoCar. Each GoCar will take about 10-20 privately-owned cars off the road. This frees up space for civic amenities such as wider footpaths, trees and parks.