It's also why many manufacturers range figures for fully electric vehicles are wrong.
Matt Watson in his reviews tells you what energy the car is using at 70mph, and what your range would be at that consumption - much less than the official figure.
As I've found with the Jazz, for the type of driving I do, I get nowhere near the official mpg/mpkw figures that are published.
As a rough guideline, for the driving I do, I can get maybe 50% of the official range of an electric car.
50mpg for a Jazz at 70mph is still good, compared to 30mpg for my accord and mid/high thirties for my civic.