Fuel economy: the real story - Introduction
"Your car's actual fuel economy and its official figures can differ wildly - but why? We tested 26 cars to find out.
Not getting close to your car's claimed fuel economy figures? It's a common problem - and there's a good reason for this: the official figures are obtained using tests that are unrepresentative of the sort of driving you actually do.
Sure, but even if you can't match the official figures, at least they provide you with a level playing field, so you can compare one car's economy directly with another's, right?
Well, that's how the argument goes, but when we asked fuel consumption expert Peter de Nayer to test 26 cars, it turned out that you need a much bigger pinch of salt with some models' real-world economy than others..."
http://www.whatcar.com/car-news/fuel-economy-the-real-story/242373