The highest-emitting petrol Euro 6 vehicle family had approximately the same level of NOx emissions as the lowest-emitting diesel vehicle family.
One involved using a portable emissions measurement system (PEMS) attached to the back of a auto, while the other assessment method uses light beams to scan vehicles' exhausts.
TRUE is a partnership of the FIA Foundation, the International Council on Clean Transportation, the Global New Car Assessment Programme, Transport and Environment, and C40 Cities, which seeks to bring transparency to the public debate on vehicle emissions and urban air quality.
Volkswagen was exposed as cheating emissions tests in September 2015. Automatic number plate recognition technology is used to determine which model of auto is being tested.
The EU diesel auto NOx emissions limit is 80 mg/km and was, until 1 September 2017, only tested by vehicle makers in laboratory conditions.
Models made in 2017 were cleaner than those made in 2016 but still produced up to four times over the limit.
NOx pollution is at illegal levels in numerous countries across the European Union including Germany and the UK. It is estimated to cause 23,500 early deaths a year in the United Kingdom, where government plans to cut pollution have been repeatedly ruled so inadequate as to be illegal.
The beam tests are continuing and will be able to single out highly polluting models in future.
In London, for example, researchers used remote sensing technology to measure the emissions from more than 100,000 vehicles during a five-month sampling campaign that stretched from November 2017 through March 2018.
In fact, some of the cars rated in The Real Urban Emissions Initiative or True Initiative test were even higher than the Euro 5 level, which has less strict margins.
"If I was a customer, I would look at these figures at the moment and have to conclude I should not buy a diesel auto", added Mock.
Petrol cars fared better, however, with all Euro 6 petrol models receiving a "good" or "moderate" rating.
Autocar has approached the TRUE Initiative for comment. "It identifies the worst performing models and regulators must act to require carmakers to clean these up".
The red category includes both cars that only slightly exceed the rating's threshold for NOx emissions (180 g/km, equivalent to the Euro 5 type- approval limit for diesel cars, which all new cars should theoretically have met for at least seven years) and gross emitters that exceed the regulatory limit by 18 times or more.
However, he cautioned that True, which averages the same models across several years, will not show when the latest model has significantly cut its emissions.
Since September 2017, new cars have had to pass a road-based test, but an European Union compromise with the industry means emissions can still legally be double the baseline limit.
Mike Hawes, chief executive of the Society of Motor Manufacturers & Traders, said: "Thanks to massive investment, each generation of vehicle is more advanced than the last, with significantly lower pollutant emissions".
Data published today by the International Council on Clean Transport (ICCT) has suggested that a majority of new diesel cars being placed onto the market do not meet emissions standards. They used data on the number of vehicles and mileage, along with government estimates of the harm caused by pollution.
Research by FuelsEurope and the Association for Emissions Control by Catalyst (AECC) also shows that the latest generation of diesel vehicles will continue to play a major role in helping reach future Carbon dioxide targets.