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Fuel Economy: claimed figures vs real world MPG

  • Claimed fuel consumption figures fail to represent real world driving
  • Energy Savings Trust recommends 15% adjustment
  • Using the figures as a benchmark is the way to go

Are you one of the many motorists out there frustrated that the company car you drive doesn’t seem able to hit its official fuel consumption figures?

The difference between real world mpg and a car’s claimed figures can sometimes be huge because driving style, car maintenance and driving conditions all have an impact on performance.

Very few of us manage to hit the official figure and it’s why the Energy Saving Trust recommends a 15% adjustment from the official mpg as a realistic target for real world mpg.

For example, if your company car’s claimed figure is 61mpg, 52mpg would be a more achievable figure.

So why are the official figures so far off the mark?

The official fuel consumption test explained

Manufacturers’ claimed fuel consumption figures come from an official test called the NEDC (New European Driving Cycle), a procedure that has been in place for around 30 years.

Manufacturers are legally obligated to carry out this test and it must be independently verified and approved by the testing authority.

So the test is repeatable, the cars are loaded onto a chassis dynamometer (a rolling road) which represents a flat road. The programmed route cycle includes periods of city driving and motorway and includes elements of idling, acceleration and braking, lasting in total for nearly twenty minutes.

To ensure that all cars are tested in the same environment, the laboratory is set to a temperature between 20-30C degrees. All engines must be rested the night before and the engine is started from ‘cold’ with measuring commencing immediately.

This cycle gives the complete emissions and fuel consumption results, published in three figues - urban, extra urban and combined. The test is usually only carried out once.

We usually refer to the combined fuel economy when reviewing cars as it's the closest we can get to an average over mixed driving.

The NEDC test is not representative of real world mpg because during the test, a car’s systems such as air conditioning and lights are all turned off. There are also no hills, traffic lights or head winds to affect the fuel consumption or emission results either.

Of course, another factor which seriously affects a car's fuel economy is the driver. People's driving styles vary massively, and things like coasting up to junctions have a huge impact on fuel economy.

What’s more, because of its age, the NEDC test might not up to speed with the latest engine technology such as stop/start, which actually switches the car off when stationary instead of simply 'idling'.

There are moves to update the NEDC test or replace it entirely so it is more reflective of ‘real-world’ driving conditions.

So what are the official fuel consumption figures good for?

The official fuel consumption figures are best used as a benchmark to compare between the company cars you are looking to pick.

Although it is not the most representative of real world driving, it is a baseline that all vehicles are driven on.

With so many different variants affecting mpg, such as driving style, terrain, weight and in-car technology, the NEDC is viewed more as an industry standard process that allows cars to be compared and measured against rather than a realistic target for you to achieve.

What affects a car’s MPG?

While one of the biggest impacts on mpg is driving style, tyres and extra weight can also add anything from 2% to 20% on your average fuel consumption. Using air conditioning or opening the windows can also make around 5% difference, while traffic conditions, weather and terrain all have an impact.

Making small changes to your driving style and applying eco driving techniques have been shown to help deliver considerable improvements in mpg. 

Click here to read our full guide to economical driving that could help you cut your fuel bills.

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