Mike,
What would you expect to see in fuel consumption between the B 200 CDI and the B 200 with the same type of driving?
I averaged 6.5 L/100 km overall (8000 km) in a Renault Scénic 1.9 dCi with 130 HP and 300 Nm of torque back in 2005. That was in mixed urban/highway/fast Autobahn driving with five up + luggage for a month. In that car, in my part of Canada, I would have been around 6.0 for sure. So, roughly 1.5 L/100 less than the B 200.
Overview: Renault - Scenic shows that the 19 Scénics in the database have averaged 6.68 L/100 km, despite the fact that half of them are the longer/heavier 7 seater "Grand Scénic" version. So that's pretty much in line with my direct experience, in difficult driving conditions. The best is 5.28 and the worst is 7.91.
I think the B 200 CDI should be capable of somewhat similar L/100 figures compared to the Renault, for an overall figure. The Renault is heavier, about the same size and has a very similar engine and aerodynamics/frontal area.
Overview: Mercedes-Benz - B-Klasse shows that the average of the 21 B 200 CDIs in their database is 7.12 - roughly half a litre per 100 km more than the Renault and the best overall average is 5.76. The worst is 7.98.
So I guess with the B 200 CDI, I would be around 6.5 L/100 km in my type of driving (which the on board computer would undoubtedly say is 6.1 or 6.0). So the fuel savings would probably be minimal, and the carbon dioxide output of the diesel would be virtually identical (within 5 g/km) to that of the gasoline car that is consuming a litre more per 100 km. Namely, 172 g/km for the CDI and 177 g/km for the gasoline car I have now.
The values I posted are also the ones used here for taxation.
After the posts on gas/diesel mileage I reset my counter and tried to use as little fuel as possible. After 58km my on board computer registered 6.3 L/100km. I don´t think lower is possible on the car, the only thing left for me to do was open up the sunroof and put out a sail. There are a lot of hills in my area, which I think can contribute to a higher gas mileage.