I think the way MPG is calculated in the European Union gives people false hope as to the economy of a vehicle. It's not the car companies fault, the European Commission came up with a test routine that everyone could follow and everyone would be tested like for like (this is what allowed VW to be a bit naughty...) The official EU tests are completed on a rolling road simulator, so there is no wind, rain, snow etc getting in the way of the test. There are no pedestrians or dogs running out in front of you so no 'real world' problems which wreck your fuel economy. I usually take 20% off the factory quoted figures to give me an idea of what the actual figure will be like. My current M Class with the 250 Bluetech diesel engine averages around 34 MPG (UK gallons @4.5L), the official figures suggest it should be around 40ish. I have averaged over 40 MPG on long runs before which is not bad for what is quite a large car, but nowhere near the factory figures.
I'll be happy if my 220 diesel GLC averages 40MPG.