In order to reduce CO2 and other emissions, the first priority is to switch off wherever possible, but there are a number of other tips for doing so. The best way is to use the car only when it is necessary. For example, instead of using it for short journeys, consider walking or taking public transport where possible. Try planning journey routes to avoid congestion, combine trips, or perhaps car sharing.
When you are considering purchasing a new vehicle, choose the most fuel-efficient vehicle of the type you need. The fuel consumption of similar sized cars can vary by as much as 45%. By choosing the most fuel efficient car in class, rather than the one with the average emissions, can typically reduce fuel consumption by up to 24%.
Source: http://switchoffandbreathe.org/what-can-i-do/