Most informed consumers have long known that, generally speaking, newer cars have better fuel efficiency and lower emissions than older models. They may also believe that purchasing a hybrid or electric car is surely the most environmentally conscious choice. However, the ongoing efforts to make cleaner, more efficient autos has been going on for decades now. From minivans to SUVs, many pre-owned vehicles now have MPG (miles per gallon) ratings that are comparable to even the newest models.
Another important consideration: the environmental impact of any given car also must account for its manufacture and disposal. In fact, the automotive industry itself has performed research indicating that somewhere between 12 and 28 percent of the total carbon dioxide released during a vehicle's lifespan comes from the assembly process and distribution to dealerships. That means that when you purchase a pre-owned car instead of a new one, you reduce your carbon footprint in two ways: you bypass the polluting effects of manufacturing and delivery, and you keep one more valuable resource from being prematurely scrapped.
Hybrid and electric cars are marvelously fuel and emissions efficient. However, with today's technologies, there are still a few significant trade-offs. Hybrid cars are more expensive to manufacture, and they often rely on lithium-ion batteries containing nickel or cobalt. While innovative, the mining practices associated with creating these batteries has been deemed extremely destructive to the environment. In 2013, the United States Environmental Protection Agency concluded that batteries that use cobalt and nickel have the "highest potential for environmental impacts" in future years. Finally, fully electric cars, while themselves emission free, still need electricity from outside, meaning their actual carbon footprint will depend on the "greenness" of the energy source.
If your car is already decades old, replacing it is probably the more sustainable choice. However, there are plenty of used cars built in more recent years that can still compete with new autos in terms of overall environmental impact.