Don't let the fuel consumption of your car confuse you: WLTP vs NEDC

From 1 September 2018, all new cars sold in Europe must publish their consumption figures according to the new WLTP homologation cycle, definitively abandoning the NEDC cycle. What does this mean? How does the use of the new fuel consumption homologation cycle affect users? Let’s take a look.

The first thing to know is that the WLTP cycle – the English acronym for “Worldwide Harmonised Light Vehicles Test Procedure” – for measuring fuel consumption and CO2 emissions was created with the aim of offering figures that more faithfully reflect real driving conditions compared with the NEDC cycle – the English acronym for “New European Driving Cycle” – which became obsolete long ago. To give you an idea: it is regulated by a directive dating from 1980.

Homologated consumption versus real consumption

Does this mean that the real consumption of my car will no longer be higher than the figure homologated under the new WLTP cycle? Unfortunately, the answer is no. The homologated consumption under the new cycle will get somewhat closer to real consumption than the NEDC cycle did, but when you go to refuel your new car, you will find that it remains very difficult to get close to the average consumption homologated by the manufacturer.

This is because the homologated consumption figures offered by manufacturers are measured on a test bench, subjecting the car to a set cycle so that a baseline can be established that allows consumption to be compared between different models. The problem is that this cycle does not reproduce one hundred percent of the driving situations we encounter in real life, although the new WLTP cycle tries to.

These are the main differences between the NEDC and WLTP homologation cycles:

As the table shows, the WLTP cycle is longer (almost twice the distance, and 30 minutes long instead of 20), the average and maximum speeds during the cycle that reproduces driving are higher, the number of stops and the time spent accelerating or braking increase, and the time at constant speed clearly decreases.

This partly solves some of the problems of the NEDC cycle when it comes to achieving consumption figures that are closer to reality. For example, start-stop systems, which switch off the engine when the vehicle stops, will no longer have such a large impact on test bench measurements compared with their operation in real driving.

The temperature at which the test starts has also been reduced to 14 degrees Celsius, which reproduces cold starts somewhat more faithfully. And WLTP takes into account more situations involving sudden acceleration and braking. Acceleration increases from 1 m/s² to 1.6-1.7 m/s². And the WLTP cycle also takes into account optional equipment that may have an impact on aerodynamics, rolling resistance or the weight of the vehicle.

Even so, there are still things that pose a problem and mean that WLTP consumption still differs from real consumption. First, the length covered by the cycle is still very short. On the other hand, at least in a first phase, tests are still carried out with the air conditioning switched off. And no matter how hard one tries, a test cycle on a test bench cannot reproduce one hundred percent of real driving.

Why are the tests done on a dynamometer? We would have to go back to the 1970s, when the first standards to regulate emissions were established, until 1992, when the NEDC cycle came into force. The initial idea was to establish a test that would allow the same test to be reproduced for all manufacturers and the results to be compared. As we will see later, this may also change in the future with the RDE emissions measurement tests in real traffic.

And what about the real consumption of my car?

The first thing to make clear is that although your car’s technical data sheet says that consumption with WLTP is higher than according to the NEDC cycle, real-world consumption will be the same. What changes is the way that consumption is measured. To give you an idea, let’s take as an example the real consumption of around 8 l/100 km measured in our test of the Opel Astra 1.4 Turbo GSI Line. These are the consumption figures for this model according to the two cycles:

As you can see, although it is still below, the WLTP consumption is much closer to the real figure than the NEDC consumption.

Will the price of cars go up with the WLTP cycle?

And here comes the first problem that the administration still has to solve, and it had better do so as soon as possible: at present, the Vehicle Circulation Tax is based on pollutant emissions according to the NEDC cycle. In Spain, cars with CO2 emissions below 120 g/km are exempt from the tax, while those emitting between 121 and 160 g/km pay 4.75%, those emitting between 161 and 200 grams pay 9.75%, and those over 200 grams are taxed at 14.75%.

The logical and expected outcome is that, since emissions under the WLTP cycle are higher, the brackets will be adapted under the new regulations to seek an equivalence so that cars do not suddenly go up in price when moving from one CO2 emissions bracket to another, as would happen with the Opel Astra in the example. In reality it is the same car, with the same engine, but under current law it would be exempt from paying registration tax based on the emissions homologated under the NEDC cycle, yet it would pay 4.75% if the law is not amended for the new WLTP cycle.

The future RDE standard that will come into force in 2019

A further step towards offering users more realistic homologation figures for consumption and emissions was taken with the tests known as RDE – the English acronym for Real Driving Emissions – which, like the WLTP homologation cycle, are mandatory for new cars from 1 September 2017, and which will be applied in several phases starting in 2019.

These tests will certify that new cars comply with the Euro 6d emissions standard, especially with regard to particulates and nitrogen oxides. Here, the main novelty is that the tests are carried out on the road and under real driving conditions, not in a laboratory. The cars incorporate a portable unit to measure emissions during the three driving phases that make up the test: urban, extra-urban and motorway. Logically, the test is harmonised by establishing certain conditions of temperature, altitude, speed ranges and distance travelled.