Fuel efficiency has been a strong selling point in the auto industry for some time, but what’s often overlooked is the amount of carbon dioxide (CO2) produced by a particular model in a given year. Fortunately, we can turn to Natural Resources Canada’s 2009 Fuel Consumption Guide to find out the most heinous eco offenders.

We’ve put together a Top 5 list of the most toxic cars, and another Top 5 of the most environmentally friendly vehicles.

Not surprisingly, vehicles with the biggest engines dominate the most-polluting vehicle list, while the most environmentally friendly cars are powered by a four-cylinder engine (some even have fewer cylinders).

Deciphering the 2009 Fuel Consumption Guide requires a bit of work: Some E85-powered vehicles are at the top of the list, but they wouldn’t be there were they ranked based on regular gas consumption (one example is the Lincoln Town Car which produces 3,040 kg of CO2 when running on E85 compared to 5,276 kg/year for the regular gas version). The other problem is that E85 is not readily available in Canada, compared to the United States where it is starting to be distributed on a larger scale. As such, the E85 figures aren’t included on our list.