Currently there is no official body in place to police the testing procedure and monitor the results taken from the different labs.
Tests conducted include simulating in town (urban) and out of town (extra urban) driving conditions, reaching a top speed of 75mph for 10 seconds. It does not included sustained motorway driving, where many cars consume the most amount of fuel.
Removing roof rails, extra lights and even the passenger door mirror is permitted. Making the car lighter results in a more impressive mpg figure.
The same test procedure is followed by all manufacturers, however, they can pick from any credited lab to do the tests. The issue here, how is it possible to compare results from multiple labs using different equipment?
The rule allows vehicle makers to arbitrarily knock 4% off the results at the end.
No restriction in tyre pressure. Higher-than-recommended pressures may be used to reduce rolling resistance, taking load off the engine and reducing fuel usage.
Modern vehicles are being introduced with adaptable driving modes, such as an ‘eco mode’ for fuel efficient driving. Manufacturers have the option to elect ‘eco mode’ rather than the default settings of the vehicle for more impressive mpg.
8. Auxillary Loads
Tests are conducted with all auxiliary modes such as air conditioning, lights, heated windows, heated seat, radio etc turned off. This results in further impressive mpg figures.
http://www.which.co.uk/news/2015/04/false-economy—98-of-cars-cant-match-their-mpg-claims-401750/
http://blog.greenflag.com/2015/car-fuel-economy-new-test-to-give-more-achievable-official-mpg/
http://www.nextgreencar.com/mpg/real-mpg/ https://recombu.com/cars/article/fuel-for-thought-majority-of-new-cars-failing-to-achieve-claimed-mpg
http://www.autocar.co.uk/blogs/tester-s-notes/vw-e-nedc-and-benefits-realistic-range-claims
http://www.jct600vehicleleasingsolutions.co.uk/unrealistic-mpg-claims/
http://www.iol.co.za/motoring/industry-news/how-carmakers-fake-economy-figures-1.1765788#.Vc3Jfrbh5z0 http://www.greencarguide.co.uk/features/new-european-driving-cycle-nedc-fuel-economy-test-work/