The old European test let carmakers remove mirrors and over-inflate tyres; the new one runs longer, faster and with the options fitted.
WLTP is the global laboratory test for the fuel use, CO2 and pollutant output of cars, plus the electric range of plug-in vehicles. It replaced the NEDC, which was criticised for unachievable figures. It is longer, faster and more dynamic, though a laboratory figure still differs from real driving.
According to the UK consumer group Which?, Europe's old fuel economy test let manufacturers strip off the roof rails and a door mirror, pump up the tyres beyond the recommended pressure and run 2 km/h below the required speed. That test, the NEDC, has been replaced by WLTP.
What does WLTP measure?
WLTP is a global driving-cycle standard that measures pollutants, CO2 and fuel consumption for conventional petrol and diesel cars and hybrids, and how far plug-in electric cars can go on battery alone. It was adopted by the United Nations Economic Commission for Europe, its final version appeared in 2015, and China, Japan, the US and the EU are among those that accept it.
Its stated main goal is to bring laboratory estimates of fuel consumption and emissions closer to what cars do on the road. A second aim is to harmonise test procedures worldwide and create an equal playing field for makers. India, South Korea and Japan use it as their standard fuel economy and emission test, alongside the EU countries.
How the test runs
The car repeats a fixed speed profile that sets one speed value for each of 1800 seconds, which makes a test of 30 minutes. Tests take place in a laboratory under clear and repeatable conditions, with the protocol fixing everything from dynamometer calibration to room temperature, air density and wind. It also fixes total car weight, including optional equipment, cargo and passengers, as well as fuel quality, tyre selection and tyre pressure.
The cycle is divided by the power-to-weight ratio of the car. Class 1 covers low-power vehicles, Class 2 sits in the middle, and Class 3 covers high-power vehicles. Most cars on sale today have 40 to 100 W/kg, so they belong in Class 3, while vans and buses can fall into Class 2.
A Class 3 test has four parts, each with its own ceiling: the Low phase tops out at 56.5 km/h, Medium at 76.6 km/h, High at 97.4 km/h and Extra-high at 131.3 km/h. They simulate urban, suburban, rural and highway driving, with a split of 52 percent urban and 48 percent non-urban.
WLTP sets no fixed gear shift points, unlike the NEDC, so each vehicle changes gear where its own weight, torque map, power and engine speed suit it best. And the optional equipment fitted to a model is counted: homologation needs one measurement for the standard car and another for the fully equipped one, to capture the effect on aerodynamics, rolling resistance and mass.
The car did not change; the test did.
What was the NEDC, and why was it criticised?
The New European Driving Cycle was the old European test for emissions and fuel economy in passenger cars, last updated in 1997. It combined four repeated urban cycles with one extra-urban cycle. It is repeatedly criticised for producing economy figures that cannot be achieved in reality.
The cycle was conceived when European cars were lighter and less powerful. It offers a stylised speed pattern with gentle accelerations, long constant-speed cruises and many idling events. Real accelerations are steeper and more variable, partly because modern engines have more power in reserve: the average 0-100 km/h time fell from 14 seconds in 1981 to 9 seconds in 2007. A Swedish researcher had already criticised the standard in 1998 for allowing large gaps between test and reality.
The average speed was only 34 km/h, with a top speed of 120 km/h.
The Which? charge sheet
The consumer group Which? called the procedure out of date, because its last update came before hybrids and stop-start technology were widely available. It also alleged that nobody official supervises the tests, and that manufacturers can arbitrarily trim their results by 4 percent at the end of the cycle. The weaknesses it listed:
- Tests are not necessarily repeatable or comparable.
- The cycle includes no sustained motorway driving.
- Optional economy settings, which drivers will not normally pick, can be used.
- Air conditioning and heated windows are switched off.
- A car can run 2 km/h (1.2 mph) below the required speed, using less fuel.
- Roof rails and a passenger door mirror can be removed to cut drag.
- Tyres can be inflated above the recommended pressure to lower rolling resistance.
A related complaint is cycle beating. Because the NEDC uses fixed speeds, gear shift points and accelerations, manufacturers can tune engines to the exact test points while emissions in typical driving come out much higher. Two German technology institutes found that diesel cars had delivered no real NOx reductions after 13 years of stricter standards. Some have alleged that makers also disconnect brakes to cut friction or tape over cracks between panels to reduce air resistance.
NEDC and WLTP side by side
| Feature | NEDC | WLTP |
|---|---|---|
| Test duration | 20 minutes | 30 minutes |
| Distance | About 11 km | 23.25 km |
| Average speed | 34 km/h | 46.5 km/h |
| Top speed | 120 km/h | 131.3 km/h |
| Speed profile | Smooth accelerations, few and prolonged stops | More dynamic, quicker accelerations followed by short brakes |
| Driving conditions | Urban and extra-urban | Urban, suburban, main road and highway |
| Gear shifts | Fixed shift points | Each vehicle uses its own optimal points |
| Optional equipment | Standard car only | Standard and fully equipped car measured separately |
Why do real-world figures still differ from WLTP?
WLTP is still a laboratory test, and it narrows the gap rather than closing it. Its CO2 figures usually come out higher than the NEDC for the same car, which is a change in the test, not in the car's performance.
Real roads add variables no cycle can fix. The energy in fuel has to overcome wind resistance and tyre drag and also feed systems such as ignition and air conditioning, and driver behaviour matters: sudden acceleration and heavy braking waste energy. Studies on steady speeds show that economy depends on how fast a car is driven, with some vehicles using less fuel at 100 km/h (62 mph) than when crawling along at 70 km/h (43 mph), though their best result lies between 40 and 90 km/h (25 to 56 mph).
Where a car drives matters too. The share of driving on high-speed roads ranges from 4 percent in Ireland to 41 percent in the Netherlands. Engines differ: passenger-car diesels reach up to 41 percent energy efficiency but typically 30 percent, while petrol engines reach up to 37.3 percent but more typically 20 percent. Diesel fuel also holds about 10 percent more energy per litre. A Skoda Octavia with a 78 kW petrol engine is rated at 5.7 L/100 km, against 4.5 L/100 km for the 78 kW diesel.
The fleet lags the showroom, too. In Australia in 2004, the average car on the road used 11.5 L/100 km, while the average new car used 9.3 L/100 km. In the United States, new passenger cars averaged 13.8 L/100 km in 1978 and 10.7 L/100 km in 1982, and the 2020 model year averaged 9.3 L/100 km.
What the real-driving test adds
Alongside the laboratory procedure, the UN body introduced a Real Drive Emissions test for NOx and particulates. It does not replace the lab test, which is the only one with legal value, but complements it. Cars are driven on real roads across different heights, temperatures, payloads, slopes and road speeds, carrying a portable emissions measurement system with gas analysers, flowmeters, a weather station and GPS.
The limits are the WLTP limits multiplied by a conformity factor that allows for instrument error. For NOx, the factor is 1.5, or 50 percent above normal tolerance.
How do you read fuel economy figures?
Fuel consumption is quoted in litres per 100 km, where a lower number is better, or as fuel economy in km/L or miles per gallon, where a higher number is better. Electric cars use kilowatt-hours per 100 km. The unit changes by country, so first identify the unit before comparing.
Europe, Australia, New Zealand, Canada, China and much of Africa use litres per 100 km, with the UK, Denmark and the Netherlands the main European exceptions. The US and UK popularised miles per gallon, and Europe, India, Japan and South Korea use km/L. Even mpg needs a check, because the imperial gallon is 4.54609 litres and the US gallon is 3.785 litres. New US cars also carry a sticker showing gallons per 100 miles.
To convert, divide 235.215 by the litres per 100 km value to get miles per US gallon, or 282.481 for miles per imperial gallon. In the US, an equivalent called miles per gallon gasoline equivalent has been created to compare electric cars with petrol ones.
Why do identical cars get different official figures?
Published fuel economy changes between jurisdictions because their testing protocols differ. The same Lexus IS 250, a petrol V6 automatic with rear-wheel drive, returned these combined figures, in litres per 100 km:
- European Union: 8.9
- Australia: 9.1
- Canada: 9.6
- United States: 9.8
The gaps show up in the split figures as well. The urban number was 12.7 in Australia, 12.5 in the EU, 11.1 in Canada and 11.2 in the United States, while the extra-urban or highway number ranged from 6.9 in the EU to 8.1 in the US. The car did not change; the test did.
What did the switch from NEDC to WLTP involve?
The transition ran from 2017 to September 2019. From 1 September 2017, manufacturers had to obtain approval under both procedures for any new vehicle, and WLTP superseded the NEDC from September 2018, after which only WLTP fuel consumption and CO2 figures have legal validity in the official certificate. From 1 September 2019, every light-duty vehicle registered in EU countries, and in Switzerland, Norway, Iceland and Turkey, had to comply.
Money was at stake: in many countries the CO2 figure feeds into the Vehicle Excise Duty bill on a new car. The UN body suggested that policymakers account for the gap. In the UK, a CO2 value measured under WLTP had to be converted into an NEDC equivalent during the transition.
Japan made WLTP mandatory in September 2021, and China swapped in a home-grown cycle, the China Light-Duty Vehicle Test Cycle. Japan also runs a variation, the Japan-WLTP, which drops the extra-high-speed phase because average traffic speeds there are lower.
Where this goes next
Since 2021, new European cars have had to carry an on-board device that records fuel and electric energy use over the car's life. This On-Board Fuel and Energy Consumption Monitoring device stores lifetime fuel consumed, distance travelled, engine fuel rate and vehicle speed, and for hybrids it adds data on charge-depleting and charge-increasing operation and the energy fed into the battery from the grid.
For manufacturers, the numbers still carry weight. WLTP ties in with an EU rule that a maker's sales-weighted new fleet must not emit more CO2 on average than a target, which is set at 95 g of CO2-eq per kilometre for 2021. The EU's earlier regulations from 2012 were aimed at an average of 130 g/km, equal to about 4.5 L/100 km for a diesel or 5 L/100 km for a petrol car.
