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Real-world MPG and emissions testing: What is it and how does it work?

WLTP and RDE tests measuring car MPG and emissions have replaced the older, less accurate systems used before

New systems of testing car fuel economy and emissions have replaced the previous New European Driving Cycle (NEDC) type approval tests designed in the eighties. The Worldwide harmonised Light vehicle Test Procedure (WLTP) and Real Driving Emissions (RDE) have brought in tougher real world and laboratory criteria for fuel economy, CO2 and NOx assessments in answer to criticism of NEDC.

WLTP and RDE economy and emission tests explained

WLTP has a more realistic drive cycle than the NEDC, which lasted for just 20 minutes and a total of 11km (6.8 miles) with strict speed, gear and even temperature controls. Testers cover over twice the distance on the WLTP test to get an official economy figure, while the average speed of the test has risen to 29mph (from 21mph), with a maximum of 81mph attained (up from 75mph).

Aside from longer test cycles and variable gearchanges, the checks require each car to be tested with its optional equipment; the base spec car is no longer taken to be representative of the whole range. Also, the WLTP test is supported by a real-world driving element to measure Nitrogen Oxide (NOx) emissions – the Real Driving Emissions (RDE) test.

All new cars on sale now have their fuel economy and CO2 emissions tested via WLTP and their NOx emissions via RDE1. Cars newly brought to market from January 2020 will have to meet the stricter RDE2 regulations on NOx emissions - these will then come into effect for all new cars on sale, including those originally brought to market before January 2020, in January 2021.

Are WLTP and RDE fair?

Although WLTP and RDE produce much more realistic results for fuel economy and emissions, meaning drivers are more likely to be able to achieve their car’s stated MPG, the new systems haven’t escaped without criticism.

The AIR (Allow Independent Road-testing) Index is an independent emissions rating system which offers a simple red, amber and green ranking system for cars based on their NOx emissions, along with a grade ranging from A to E.

The scientists behind AIR believe there’s an inherent problem with WLTP and RDE, which is allowing the car industry to continue regulating its own fuel economy and emissions figures. They argue these should be tested and graded independently.

One such scientist is Dan Carder, director of the Centre for Alternative Fuels, Engines and Emissions at West Virginia University. He led a team that published some of the earliest evidence of the Volkswagen Dieselgate emissions scandal.

When AIR launched in February 2019, Carder said: “If the AIR Index had been implemented 15 years ago, Dieselgate would likely not have happened.

“Until the recent implementation of the AIR Index, the air quality debate was devoid of objective, independent, and publicly available assessment of vehicles’ actual NOx emissions during real driving in urban environments.”

WLTP and RDE vs NEDC: Car MPG and emissions tests compared

 NEDCWLTPRDE
LocationLaboratoryLaboratory On the road
Cycle length20 minutes30 minutesBetween 90 minutes and 2 hours
Route length11.03km (6.85 miles)23.27km (14.45 miles)At least 48km (29.82 miles)
Share of test spent   
accelerating/decelerating36%84%N/A
Share of test spent   
at constant cruising40%4%N/A
Share of test spent idle24%13%N/A
Average speed 33.6kph (20.87mph)46.5kph (28.89mph)Urban (15-40kph/9-24mph)
   Rural (60-90kph/37.28-55.92mph)
   Motorway (>90kph/55.92mph)
Maximum speed120kph (74.56mph)131.3kph (81.58mph)Motorway driving has to have at least 5 mins at >100kph (62.13mph)
    
GearshiftsFixed gear positionsVehicle-specific gear positionsVaries

What do you think about the real-world car emissions testing issue? Let us know in the comments...

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