Rated 4.8 out of 5 on 
The NEDC test defines the official mpg figures, the ones car manufacturers require in order to show on their marketing. This is a requirement to conform to relevant regulations. The only problem, the test is outdated and no longer fit-for-purpose. The last time the test was updated, was back in 1997, which no longer accounts for modern engines that include such technology as hybrid power, adaptive driving modes and start-stop features. As a result the test does a poor job on reflecting real-world driving conditions. A new test was due to replace the current one in 2017. However, due to delays it is looking more likely it will be ready by 2020. Which? are now calling for it to be ready as initially agreed… and we support them for this.
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.
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.