Remember that the EEC fuel consumption test that's used in Europe and in much of the world, and is similar to the EPA test used in the USA, is a simulated test. The vehicle in question isn't driven on the road but is put through simulated driving cycles on a rolling-road dyno. The driving cycles simulate 'urban' and 'extra-urban' driving via a series of accelerations, decelerations, constant speed running, idle periods etc. The test wasn't devised by BMW, or any manufacturer for that matter, it's the work of government bureaucrats designed to 'help' the consumer. So when BMW says it cars get Xmpg or use X litres/100km, this is simply the figure it achieves in the relevant EEC or EPA test. BMW, like all manufacturers, goes out of its way to say that real-world fuel consumption will vary from these figures.