This is pretty cool, this UK website rates cars based on CO2 output per kilometer.

We've teamed-up with What Car? to make it easy to find out which top ten new cars have the lowest CO2 in their class. We have used What Car?'s classification system to define which class (for example supermini, small family, executive) a car is in, to make it easier to find the type of car you are looking for. The aim of this ranking system is not to recommend specific models but to give you information that can help you make a CO2-informed decision. You can refine your search by fuel and gearbox types, and to make life even simpler, we've colour-coded each entry to reflect the Fuel Economy Label – it's like the one you get on fridges and washing machines.