Official fuel consumption figures - why they're nonsense

More than ever, drivers are focusing on their car’s fuel consumption, in a bid to save money. It’s no wonder; squeezing an extra few miles out of each gallon can make a big difference to a car’s running costs.

How you drive a car makes a big difference to what sort of fuel consumption you get from it, but for many, the assumption is that it should be easy enough to achieve the official fuel consumption figures. 

How you drive a car makes a big difference to what sort of fuel consumption you get from it

The problem is, those figures are achieved on a rolling road in a laboratory, under test conditions – and to say they don’t reflect the real world is a massive understatement.

Known as the New European Driving Cycle (NEDC), these fuel economy tests are made up of two parts. The first requires a car to be accelerated from a standing start to 43mph in 45 seconds, then settling down to a short cruise, with the car averaging 39mph. This is meant to simulate the extra-urban (out of town) driving that you do.

Next comes the urban test, which sees the car accelerated to 10mph, then 20mph, then 30mph, in its lowest three gears. Fair enough you may say – but a whopping 195 seconds (more than three minutes!) is taken to do this. The procedure is repeated another three times, with idling time allowed between each one.

So if you accelerate more slowly than a pedal cyclist and sit at 39mph on the open road, you can expect to achieve the official fuel consumption figures quoted by the manufacturer of your car - although the drivers you inconvenience on every trip will probably catch up with you and bludgeon you to death fairly soon. Meanwhile, for those of us in the real world, we’ll never get even close to what’s claimed.

As if the tests aren’t unrealistic enough, car makers are also tuning their cars to do well in these specific laboratory cycles, ensuring those on-paper figures are even further removed from reality. And to make things even worse, with the tests all performed on a rolling road, there’s no wind resistance to take into account – unlike when you’re steaming along the M40 at 70mph.

Because all new cars have to go through these tests, at least you can make direct comparisons between the various models you’re considering. But bigger cars tend to be able to get closer to the official figures than city cars and superminis – so if you’re buying a Mercedes S-Class rather than a Fiat 500 (for example), you may be less disappointed by the fuel economy you can achieve. Although if you’re loaded enough to buy an S-Class, you’ll probably be too minted to care.

* Want to drive more economically? Then check out our guide to eco driving.