There are a few reasons for the difference.<br> <br> Mainly, the BMW engine is a more efficient design, as it has four valves per cylinder. The Mercedes V6 only has 3. The single exhaust valve per cylinder has the advantage of keeping exhaust temperatures high (cleaner exhaust) but the disadvantage of asthmatic breathing. Mercedes said a few years ago that four valve per cylinder engines would be a thing of the past due to tightening (EU-4) emission control standards, but BMW and nearly every other carmaker have proven them wrong. I expect the upcoming direct injection gas engined Mercedes-Benz V engines to have four valves per cylinder (just my guess though).<br> <br> As another poster pointed out, the BMW is slightly lighter and produces less power, which can also explain some of the difference.<br> <br> The official fuel consumption figures say 16.1 L/100 km in the city and 7.8 on the highway. Your highway mileage sounds a bit poor. But if you are doing pure stop-and-go driving in the city, the mileage sounds about right. Ask the dealer to check it out just to be sure.