This follows an earlier Guardian report that revealed substantially higher levels of pollution in diesel vehicles sold by Renault, Jeep, Hyundai, Fiat, Citroen, and many others.
The first round of data was compiled by ADAC, Europe's largest motoring organization, which tested vehicles first using the EU's lab-based testing (depicted above), then compared the data to a longer, UN-developed lab test that's thought to more realistically mimic real-world driving. The second report used on-road tests conducted by Emissions Analytics.
It's worth pointing out that all the vehicles tested were European-market diesels, many of which are not available in the U.S., and all of which passed either the current Euro 6 or the previous Euro 5 standard in the EU's official lab-based testing.
It's also important to note that none of the vehicles tested are accused of having illegal "defeat devices," as is the case with the Volkswagen Group diesel vehicles currently under investigation in the U.S. and Europe.
But The Guardian's reporting points out something that's been feared ever since emissions testing began: lab-based tests often do not accurately measure how a car performs in real-world use, and automakers often engineer their products to meet regulatory tests while emitting far more than the legal limit when driven in the real world.