WLTP: What it means for MPG figures
WLTP testing is designed to deliver more realistic efficiency figures, but does it? Our complete guide to WLTP explains what it is and how it affects you.
- WLTP testing's methodology explained
- Why more realistic mpg and mi/kWh figures result
- WLTP's impact on CO2 emissions explored
If you've scrolled down car manufacturers' websites or flicked through their brochures recently, the chances are you will have spotted reference to WLTP — it hardly trips off the tongue but it's less of a mouthful than Worldwide harmonized Light vehicles Test Procedure.
WLTP's overall rationale is to more closely align the complex array of different legal standards cars must comply with in various parts of the world, such as exterior light positions, seatbelt buckle types and overall efficiency.
The more countries and trading blocs that adopt WLTP protocols, the more cost-effective it is for car manufacturers to sell their products there.
For the UK — which still largely follows EU regulations when it comes to vehicle requirements — it's WLTP's fuel economy (mpg), electrical efficiency (mi/kWh) and CO2 emissions (g/km) standards that matter most to car buyers.
In theory, the manufacturer-quoted figures on their websites and brochures should be closer to what you can expect in the real world under WLTP — however, they are a guide and not a promise, which has always been the case.
When was WLTP introduced?
WLTP testing protocols were phased in their introduction to allow car manufacturers — and governments — plenty of time to comply with the changing requirements from the previous method known as the New European Driving Cycle, or NEDC for short.
Newly launched car ranges were tested to WLTP standards from September 2017, with pre-existing models switching across in September 2018.
Because the aim of WLTP testing was to provide more realistic mpg and mi/kWh figures the published numbers would typically be lower than those from NEDC tests. To avoid confusing consumers who may wrongly assume that cars tested under the new rules were less efficient and best avoided, mpg and mi/kWh figures for both NEDC and WLTP were equalised, labelled as NEDC equivalent or eNEDC.
This rule remained in place until January 2019, at which point all fuel economy and electrical efficiency figures for newly sold cars swapped to WLTP.
It's important to remember that where a car derivative sold throughout NEDC, eNEDC and WLTP economy figures being published appears to have become less efficient, it's only because the testing methodology became more stringent. In the real world the economy level you achieved while driving the car won't change.
What are the differences between NEDC and WLTP?
In reality, NEDC testing wasn't fit for purpose and needed replacing — its overoptimistic efficiency figures were one of the reasons we launched Real MPG, to give drivers realistic mpg data.
Based on a methodolgy said to mimic typical European driving scenarios introduced in 1970, NEDC's most recent update was in 1997 when the previous Urban, 56mph, 75mph efficiency bands were replaced with Urban, Extra-Urban and Combined.
Although NEDC Combined figures were closer to reality for most drivers than before, they were far from close enough.
Banding for the WLTP tests are Low, Medium, High, Extra-High and Combined, with an additional City category for EVs and PHEVs.
Just as with NEDC methodology, WLTP testing takes place in laboratory conditions and consequently doesn't take into account variations such as weather, congestion and driving styles. However, WLTP is influenced by a vehicle's weight, aerodynamics and rolling resistance.
NEDC | WLTP | |
Vehicle starting temperature | From cold | From cold |
Cycle time | 20 minutes | 30 minutes |
Idle time | 25% | 13% |
Cycle distance | 6.8 miles | 14.5 miles |
Average speed | 21mph | 29mph |
Maximum speed | 75mph | 81mph |
Ambient temperature | 20-30C | 14C |
Vehicle level tested | Each engine, gearbox, bodywork combination — options not tested | Each individual derivative, with option combinations tested separately |
Why WLTP means fewer trim and option choices
Due to WLTP being more granular than its NEDC predecessor, car manufacturers are required to subject each individual derivative for economy and efficiency testing.
As each test has a cost implication, most manufacturers have reduced the number of variants in each of their ranges since WLTP's introduction, resulting in a reduced choice for consumers.
It's a similar story with extra-cost optional extras — pre-WLTP brochures and price lists would be bristling with all manner of ways customers could personalise their new cars but not now.
As each option will have a given weight and will therefore influence the car's overall efficiency, every individual extra — and combinations of them — would require WLTP testing. The result is a restricted range of available options, with many manufacturers bundling them together as packs.
Some have stopped offering optional extras altogther, leaving customers only to choose the trim level and colour they want their car to be painted.
Occasionally, mpg and mi/kWh figures may alter where a car's equipment level appears to be unchanged. This is usually due to changes in component specifications or even swapping to a different supplier, as often happens with tyres.
One tyre company's rubber will match the size of another's, but there will be differences in weight and rolling resistance, with new tests being mandated.
Are WLTP figures more realistic?
They are certainly more realistic than those produced from the old NEDC testing but most drivers still find their average economy is lower than those published by their car's manufacturer.
There are many reasons for this, not least that for uniformity, the tests have to be undertaken in controlled conditions, something that's impossible to replicate outdoors on real roads.
Not only will the road surface impact a car's actual mpg or mi/kWh figure, the temperature, humidity, number of passengers, weight of luggage in the boot, tyre pressures, the driver's idiosyncracies and the routes driven are among many factors that will vary real world efficiency.
While they are not breaking any legislative rules by doing so, car manufacturers will seek to gain a competitive advantage over rivals by gently optimising their models to perform well in the official tests, such as by tweaking gear ratios to deliver improved efficiency at the various speeds tested.
Such optimisation is one reason why economy figures for plug-in hybrids (PHEVs) are so high. Yes, for those who predominantly — or only — use electrical power when driving PHEVs, it will regularly prove possible to even exceed the published figures. However, making frequent use of the engine, especially with little electrical assistance, will convince you the published mpg figures are pure fantasy.
Does WLTP affect CO2-based tax?
Since 1 March 2001, Vehicle Exise Duty (VED) — often unofficially referred to as road tax — has been based on cars' CO2 emissions.
Originally based on NEDC test figures, it transitioned to WLTP testing-based NEDC equivalent (or eNEDC) results during 2018. In most instances, this caused the published CO2 figure to increase, potentially raising the rate of VED as a consequence.
Actual WLTP testing CO2 figures were only used for VED purposes from April 2020, causing further increases in the numbers published and VED cost as a result.
Is RDE linked to WLTP?
No, RDE — short for Real Drive Emissions — is a complementary test that vehicles are subjected to, which is separate from WLTP.
RDE testing is used to determine the level of pollutants cars fitted with petrol and diesel engines emit from their exhausts, such as NOx and particulates.
As with WLTP, its introduction has been phased — RDE step 1 was applicable to all new car ranges from Septmber 2017 and then to all existing models sold new from September 2019.
More stringent measures were introduced with RDE step 2 which applied to all-new ranges from January 2020 and all existing models sold from January 2021.
Diesel cars are particularly affected by RDE's introduction with almost requiring an additional system known as Selective Catalytic Reduction (SCR) to be fitted in order to pass the test. This involves cleaning the exhaust emissions with a detergent called AdBlue before it exits the tailpipe.
Check what MPG you should be getting with Real MPG