The Smoking Gun: emissions-rigging scandal uncovered in December last year by BBC's Newsnight programme
The BBC report from December 2014, reveals how car efficiency in the real world - that is, when driven by your average motorist - is normally always 22pc worse than advertised.
The car expert then tells the BBC about a number of tweaks that manufacturers across the industry regularly use to boost test numbers without falling foul of the rules:
-They use ultra slick tyres not suitable for a road
-They pump tyres to a very high pressure often using special gas
-They use very expensive lubricants to make the engine more efficient
-They even get rid of the offside wing mirrors as this is not a legal requirement for a test
-Finally, the expert says: "Cars can detect being tested. They can use that to put the car into a mode in which the engine is ultra efficient or to reduce the sort of pollution coming out of the exhaust pipe during a test...the car knows it is being tested"

https://www.youtube.com/watch?v=pjUr3RQRERM