If you’re a regular reader of Autocar, you’re probably well-versed in the abbreviation WLTP. Worldwide Harmonised Light Vehicle Test Procedure is a new way of measuring vehicles’ fuel economy and emissions, and it has appeared on our pages for a number of reasons.  First, there’s been plenty of related news. For example, Audi sales in the UK fell by a fifth last year, due to WLTP. Porsche has almost halved its sales so far this year, also due to WLTP. Plus, we’ve started to future-proof our first drive reviews and A-Z data pages by quoting only WLTP fuel economy and CO2 figures where possible, rather than those measured under the previous NEDC cycle.  It’s easy to glaze over this stuff, but it’s important for car buyers because it can affect how much money’s in your pocket. What fuel consumption and CO2 figures should you be looking at to save a few quid or work out your company car tax: NEDC, WLTP, RDE? Confused? Then read on…  What is WLTP?  WLTP is a new testing procedure aimed at producing more real-world figures for fuel consumption and emissions than the outgoing one, NEDC (New European Driving Cycle). NEDC had been in play since the 1980s and, despite minor updates along the way, became outdated due to changes in technology and driving conditions.  Meanwhile, WLTP has been under discussion since 2008, with plans to finalise its regulation by 2018 and launch it by 2023. Then the Dieselgate scandal happened and the European Commission decided to accelerate the process, which gave car makers just 13 months to overhaul their models and complete all testing. WLTP became mandatory from 1 September 2018 for measuring CO2 emissions and fuel economy on all new cars in the EU. Confusingly, in the UK, labelling regulations meant that WLTP fuel economy values were introduced from 1 January 2019, but CO2 figures are not due to be disclosed until 1 April 2020 to tally with changes in taxation. So while all cars will have a WLTP CO2 figure, in many cases it has yet to be publicised.  As a result of this shortened time frame, manufacturers fell into trouble last year as demand at test centres bottlenecked. It meant many new vehicles weren’t ready to be sold from September, which sucker-punched the car market. Particularly hard hit were Volkswagen Group models. In September, Volkswagen’s European sales fell by 53% and Audi’s dropped by 60%. The problem was exacerbated by a crucial difference between WLTP and NEDC. WLTP requires significantly more testing to provide individual data for every variant of a vehicle, which wasn’t needed for NEDC. If you’ve noticed that the number of options on cars have been dramatically reduced and equipment ‘packs’ have increased, that’s why. How does the test work?  The WLTP test procedure looks identical to the NEDC process, carried out on a rolling road in laboratory conditions, but that’s where the similarities end. While the WLTP test itself takes only 30 minutes, as opposed to the NEDC’s 20, the whole process can take up to five days, more than double that of its predecessor.  To reflect real-world conditions, temperature is set at 23deg C and, given that warmer temperatures put less load on a vehicle, there is another test for European vehicles at a more representative 14deg C.  There is sharper acceleration and deceleration than previously, mirroring busier traffic conditions; higher average and maximum speeds; and five driving cycles, compared with the NEDC’s three, again to reflect modern-day driving.  The other major difference concerns gearchanges during the test. In NEDC, there were fixed points for gearchanges; in WLTP, there are different gearshift points for each vehicle.  So how does all of this affect the figures? Our sister title What Car? has found that the average gap between WLTP fuel economy and its own real-world True MPG test data is 4.9%, compared with a difference of 23.5% under the NEDC cycle.  That’s WLTP in a hard-to-crack nutshell. But there’s something else: RDE. The Real Driving Emissions test, also introduced in September 2018, measures particulates and nitrogen oxides and backs up the results of a WLTP test. It takes place on public roads with a portable emissions measurement system. The 1.5- to two-hour test (over around 50 miles) has an equal split of urban, rural and motorway routes, takes into account stops/starts in towns and must exceed 56mph on appropriate roads. Essentially, it’s a validation exercise for WLTP results (which also note NOx and particulates).  The goalposts for RDE will keep moving, too. For example, at the moment, manufacturers must sign a legal document stating that the RDE results can be achieved in certain conditions, such as an altitude of up to 700 metres. From 2021, that will extend to 1300m. The gap between the WLTP and RDE figures must also decrease over time.  From 2020, a second step, RDE2 – which means lower NOx levels than are acceptable today – will become compulsory.  Why should I
Origin: The Autocar guide to WLTP emissions testing