New WLTP and RDE fuel economy and emissions tests explained

WLTP test cycles are based on real-world driving data collected worldwide. The new cycles are split into four parts, based on low, medium, high and extra high average speeds. Each speed range will comprise a variety of phases, including stops, acceleration and braking. The test cycles are longer at 30 minutes instead of 20 minutes and the highest speed is now 81.4mph rather than 75mph. Where NEDC relied on fixed gearshift points, WLTP specifies different shift points for each vehicle.

NEDC fails to take into account options such as larger wheels or any items that increase the standard kerb weight. Neither does it account for what is now common standard equipment such as heated seats, air conditioning, four-wheel drive and heated windows. WLTP takes all of those factors into consideration.

Where does the Real Driving Emissions test figure in all this?

However improved WLTP is, it is still a laboratory test and cannot take into account whether an individual driver’s style is aggressive or sedate, the traffic conditions, weather conditions, gradients or load on the car. All of these things have an effect on fuel consumption and NOx emissions. The RDE test is carried out by fitting Portable Emissions Measuring (PEMS) equipment to the car to record exhaust emissions at the tailpipe. 

The test takes place on a route that ticks the same boxes whatever the location, including high speed on motorways, low speed in urban conditions, medium speed on rural roads, low and high altitudes, driving up and down hills, increased payloads in the car and different climatic conditions covering year-round temperature and humidity. The results will be used in conjunction with the laboratory-controlled WLTP tests to calculate the final figures for each car.

How will the new tests be introduced?

RDE will apply to cars being Type Approved for the first time from 1 September, then all new registrations from September 2019. WLTP also affects new types of cars from 1 September but, from a buyer’s perspective, is likely to make things more complicated. Cars approved before that date will continue to use official CO2 values measured under NEDC only. WLTP CO2 figures will almost certainly be worse than the NEDC equivalent, so both sets of official figures will be given for cars certified after 1 September. From September 2018, cars will only be given WLTP CO2 values and that’s when comparisons could become more difficult, although manufacturers may publish unofficial NEDC figures in their own literature in an attempt to clear up any confusion.

Will the new tests make things better?

RDE tests will be subject to a 'conformity factor' - essentially 'wiggle room' allowing a margin of error on results to take into account the fact that, at present, the performance of PEMS equipment isn’t consistent. One expert we spoke to partly puts this down to the effect of installing sensitive equipment originally designed for use in laboratories in the back of a car. This will allow emissions levels to be exceeded by 110% (2.1 times) for new types of car from 1 September 1. That figure will be reduced to 50% from September 2019 and apply to all cars. So, for example, this means a diesel, which should meet an NOx target of 80g/km, could be allowed up to 168g/km in an RDE test until September 2019. 

Although that seems lax, these are ‘not to exceed’ figures and, in reality, the average test results are expected to fall well below today’s NEDC. Because manufacturers have no control over the location and conditions that a given RDE test will be carried out in, they will have to ensure that NOx levels fall well below the allowed limits to be certain of passing the test. The good news is that, from now on, the spectre of NOx in particular should vanish from the roads as real-world tailpipe emissions in new cars plummet. It also means that official fuel consumption figures should be far more representative of what we can expect when we take a car home from the showroom.