The declared fuel consumption of cars, and the levels of CO2 emissions, are those obtained during homologation in accordance with the WLTP test cycle, but everyone now knows that they are a long way from the actual fuel consumption seen on the road.

Why is this? How do the WLTP certification cycles for fuel consumption and CO2 emissions actually work? To what extent are the homologation figures lower than those for real-life driving? To answer these and other questions, we decided to delve a little deeper into the world of the WLTP cycle.

WLTP: laboratory tests to compare cars

Let's start by saying that the WLTP homologation cycle, which is compulsory for all cars registered from 1 March 2019, is an improvement on the old NEDC cycle, at least in terms of the difference with actual consumption.

Laboratory fuel consumption and emissions test (WLTP)

Laboratory fuel consumption and emissions tests (WLTP)

The fact remains that, because these are laboratory tests, the fuel consumption figures that appear on the type-approval sheets for all new cars are almost always lower than those that any motorist can achieve on the road. Even the EU's partial introduction of on-road emissions testing using the RDE (Real Driving Emissions) system does not seem to have solved the problem.

In defence of the WLTP cycle, designed for worldwide distribution at the instigation of the United Nations Economic Commission for Europe (UNECE), it has to be said that these laboratory tests are currently the only way of properly comparing the fuel consumption of different cars, precisely by following clear protocols and conditions that can be repeated.

Four cycles on the rollers at different speeds

More concretely, WLTP consumption data is obtained by placing the car on dynamometer rollers in a test chamber at a temperature of 23°C. The car is tested during a 30-minute cycle divided into four phases at different speeds simulating different driving conditions: low, medium, high and very high, i.e. urban, suburban, extra-urban and motorway.

The fourth stage of the fuel consumption and emissions test cycle (WLTP)

The four phases of the fuel consumption and emissions test cycle (WLTP) [source : ]

The maximum speeds achieved during the four phases of the class 3b WLTP test (the one that concerns the majority of cars on the market) are respectively 56.5, 76.6, 97.4 and 131.3 km/h, separated by cyclic stop periods. All this over a distance of more than 23 km, as summarised in the table below, which details the image seen above in figures.

  Low Medium High high
Duration 590 s 433 s 455 s 323 s
Duration of stop 145 s 47 s 29 s 6 s
Distance 3.095 m 4.756 m 7.162 m 8.254 m
Percentage of stops 24,6% 10,9% 6,4% 1,9%
Maximum speed 56.5 km/h 76.6 km/h 97.4 km/h 131.3 km/h
Average speed without stopping 25.0 km/h 44.4 km/h 60.5 km/h 93.7 km/h
Average speed with stops 18.9 km/h 39.5 km/h 56.7 km/h 92.0 km/h
Acceleration min -1.47 m/s² -1.49 m/s² -1.49 m/s² -1.21 m/s²
Max acceleration 1.47 m/s² 1.57 m/s² 1.58 m/s² 1.03 m/s²

Air conditioning off

It's also worth mentioning that during roller operation, all energy-consuming accessories are deactivated. This excludes the consumption of important functions such as air conditioning, heating, headlights, windscreen wipers, rear window defrosting, heated or cooled seats. However, these functions are often activated during normal driving on the road and affect, even if only slightly, actual fuel consumption.

Laboratory fuel consumption and emissions test (WLTP)

The interior of a car during laboratory fuel consumption and emissions testing (WLTP)

The WLTP cycle, meanwhile, takes into account the impact on fuel consumption and emissions of optional equipment that can add weight, alter aerodynamics or rolling resistance, such as body kits, different tyre sizes and other accessories. This is why each car is tested in the homologation cycle both in its standard version and with all the options available.

The problem of plug-in hybrids

Plug-in hybrid cars are a special case. They are tested to WLTP standards in two separate operating cycles. The first is the "charge depletion mode" or CD, in which the car operates in purely electric mode with a 100% charged battery. The second is 'charge retention mode' or CS, in which the PHEV car runs on a discharged battery using the combustion engine.

Plug-in Auto-Ibrida

Voiture hybride rechargeable

A formula combines these two results by adding a "utility factor" (Implementing Regulation (EU) 2017/1153) for the estimated share of electric mileage and gives the combined WLTP consumption, which is often very low and in any case lower than any motorist can achieve on the road.

WLTP fuel consumption for PHEVs: actual fuel consumption is up to five times higher

To give just a few examples, here are the WLTP consumption figures declared by certain plug-in hybrid cars and the results obtained with these same cars in our real-life consumption test, which takes place precisely on the road, over a standard 360 km journey, starting from a full battery (and paying attention to consumption).

  WLTP max consumption Actual consumption Difference
Kia Niro plug-in hybrid 1.0 l/100 km 3.1 l/100 km +210%
Toyota Prius plug-in hybrid 0.7 l/100 km 3.4 l/100 km +385%
Mercedes E 300 de 4MATIC Plug-in Hybrid All Terrain 0.9 l/100 km 3.8 l/100 km +322%
Peugeot 3008 Hybrid4 300 e-EAT8 1.5 l/100 km 4.5 l/100 km +200%
Jeep Compass PHEV 4xe Trailhawk 1.9 l/100 km 5.35 l/100 km +181%
Range Rover Evoque P300e Plug-in Hybrid 1.5 l/100 km 6.15 l/100 km +310%

As the table shows, the actual fuel consumption we recorded with the latest plug-in hybrid cars is significantly higher than the peer figures, ranging from three to almost five times higher than the WLTP standard.

Toyota Prius Plug-in Hybrid (2023), fuel consumption test

For electric cars, however, the same WLTP certification cycle is used to define only electricity consumption in kWh/100 km and the range achievable with a full battery.

Where is the WLTP cycle recognised?

The WLTP (Worldwide Harmonized Light Vehicles Test Procedure) cycle is a global standard now recognized by :

the European Union
South Korea
New Zealand
United States of America
South Africa

The WLTP certification cycle

The WLTP type-approval cycle

Other type-approval standards relating to fuel consumption and emissions, such as EPA in the United States, CLTC in China, Bharat in India and PROCONVE in Brazil, remain in force throughout the world.

Gallery: Laboratory consumption and emissions tests (WLTP)