Re: The VW Diesels pass the emissions test
"Agreed. They passed the tests. That much is fact.
Also a fact, they don't emit levels of NOx when driven that match the tests."
The test value can only be a marker of the possible emissions in real driving conditions allowing (if no cheating) some comparison of make/model and some rough prediction of likely effects on short-term air pollution. Given how fuel consumption varies over a large range with such variables as length of journey, stop start driving, high or moderate steady speeds and so on how could it really be anything else.
The 'easy' alternative would be to issue the worst case measurement for any model and that wouldn't reflect real-life use either.
After all take Euro6 - the test value is 80 milligrams of nox /km for cars whereas the environmental limits of nox are concentration measurements so are not directly correlatable - put another way it's some function of other sources of nox + vehicle density & usage pattern ( & wind speed/direction & rain ) for any location