Car makers are only allowed to quote three official fuel consumption figures under UK law. The Urban figure is supposed to represent city driving, Extra-Urban is for out of town use and Combined gives you an average of the two. These official figures are measured using standardized tests, so in theory they are a good way of comparing one car to another. But they are often a poor guide to what a car will really cost to run, a new technologies in cars are making the figures less and less useful.

The test distance is very short, for instance, which favours a plug-in hybrid vehicle. These charge up from grid electricity before a journey and only use petrol power once the battery is exhausted, so they return spectacular mpg figures in the official tests. Drive them over a longer distance and you will find that their overall fuel consumption gets progressively worse as the journey lengthens and their plug-in advantage diminishes.

The tests also fail to match real-world road conditions: the average and maximum speeds are low, and the acceleration required is gently. So the new generation of downsized, turbocharged engines return great mpg figures when driven very gently through the official test cycle but tend not to be so economical when pushed harder in everyday motoring conditions.

So take the official figures with a pinch of salt, particularly if you do long journeys or ever drive in a less than sedate manner. Use them to compare a car with another of the same type, but be wary of making comparisons between conventional and hybrid cars using the official figures.

And don't expect your car to return the same figures in everyday use – or you'll probably be disappointed.