Germany’s Oko-Trend Institute for Environmental Research Bergisch University of Wuppertal has declared the Mercedes S 250 CDI BlueEFFICIENCY and the ML 250 BlueTEC 4MATIC the most environmentally friendly cars in their class.
OkoTrend has published an environmental ranking for cars on an annual basis since 1997, and before reaching their conclusions, they go through tens of thousands of data relating to around a thousand car models in twelve categories. The data is related to vehicle consumption, emissions, manufacturing, logistics, recycling and environmental systems.
OkoTrend’s conclusion regarding the Mercedes S 250 CDI BlueEFFICIENCY and the ML 250 BlueTEC 4MATIC led to specialist publication AUTO TEST awarding both the luxury class model and the SUV in the "AUTO TEST WINNERS in GREEN!" and in case you are not familiar with the engine powering these two vehicles, we are talking about a 4-cylinder diesel unit featuring direct injection and turbocharging.
This 2143 cc engine delivers 204 HP (150 kW) and 500 Nm of torque and enables the S 250 CDI BlueEFFICIENCY  to cover 100 Km on just 5.7-litres of fuel while emitting just 149 grams of CO2 per Km.
On the ML 250 BlueTEC 4MATIC the same engine enables consumption and emission figures of 6.0-litres per 100 Km and 158 g/Km. 

Source: Mercedes