What is WLTP?

If you're looking to buy a car and have been browsing manufacturers' websites or downloading their brochures, the chances are that you’ll have seen the letters WLTP at various points in the technical specifications.

What those websites and brochures don't tend to do is explain what WLTP actually is and – more importantly – why it's a vast improvement over what came before.

What does WLTP stand for?

Although WLTP doesn't exactly trip off the tongue, those four initials are far more convenient than saying Worldwide Harmonised Light Vehicle Test Procedure each time it's referenced. It isn't snappy, but let's be thankful the powers that be didn't push for WHLVTP.

WLTP are a set of global technical regulations that car manufacturers must adhere to if they intend to sell their models in countries and trading blocs which decide to base their vehicle legilsation around them.

Those regulations include all manner of specifics including the minimum and maximum heights from the ground of cars' lights, their colours and brightness levels, but it's most commonly referred to in the UK and across the EU in relation to fuel and electrical energy efficiency. 

A brief history of fuel and energy efficiency testing

It was only from the late-1970s that car manufacturers were legally required to publish fuel efficiency figures across the former European Community – even then, diesels and all-wheel drive cars were initially exempted.

Their introduction enabled consumers to make more informed choices when car buyer as the fuel efficiency data enabled direct comparison between different models, albeit with the tests conducted in lab conditions rather than in the real world.

That the fuel efficiency results of the New European Driving Cycle – or NEDC – testing were near-impossible for motorists to replicate, despite methodology updates over the years, meant a more realistic alternative was sought.

It was also needed due to the growing numbers of hybrid and electric cars available as well as many EU nations basing their VED car tax equivalents on CO2 emissions, which NEDC also provided from 1997.

As a result, WLTP was selected as the methodology, requiring all new cars registered in the EU plus Iceland, Norway, Switzerland, Turkey and the UK, to be tested to the standards from 1 September 2018, with individual nations determining when the results would be legally published to replace the former NEDC figures.

In the UK, WLTP's introduction was staggered. Petrol, diesel and electrical efficiency results for the WLTP Combined cycle – representing an overall figure from the car being operated at a wide range of speeds – were introduced from 1 January 2019, but the NEDC's CO2 figures remained in use until 1 April 2020, tally with taxation changes.

How do WLTP figures compare with NEDC ones?

Because they rely on completely different testing methodologies, you can't directly compare WLTP with NEDC results – the same is also true with the various updates within the procedures during the NEDC period. 

What is noticeable are that each time those procedures change and when the switch to WLTP was made, the official resuts appear to be worse than before – and that's a good thing. Why? Because it means that the economy figures manufacturers are legally obliged to publish are becoming closer to what you might expect to achieve in the real world. 

That's important to remember because how efficient any given car is for its owner will be relatively constant over their regular commute, regardless of whether it was tested to NEDC or WLTP methodologies, so whichever official test gets closer to reality is generally better.

How does WLTP testing work? 

Whether the car being tested is combustion-engined, a mild or self-charging hybrid, or fully electric, the WLTP testing procedure is largely the same. Plug-in hybrids are tested differently, so we've outlined those separately below.

WLTP testing is similar to the former NEDC processes in that they continue to be carried out on rolling roads in laboratory conditions rather than out and about in the real world. This ensures fairness when comparing different models because consumers know the tests were performed in identical conditions, regardless of where they were undertaken.

At 30 minutes, the WLTP test is 10 minutes longer than the last NEDC type, with the exterior temperature set at 23C. There are four WLTP driving phases, each with sharper acceleration and deceleration than was used in NEDC tests, with different maximum speeds involved:

• Low cycle: urban driving up to 35.1mph

• Medium cycle: suburban driving up to 47.6mph

• High cycle: rural driving up to 60.5mph

• Extra-High cycle: motorway driving up to 81.6mph

A weighted average of those is used to determine the WLTP Combined cycle figure, which is the main result manufacturers are legally required to publish for fuel and electrical energy efficiency as well as CO2 emissions.

There's also an additional Real Drive Emissions (RDE) test to measure for nitrogen oxides (NOx) and other exhaust particulates. It's essentially a real-world backstop series of tests, carried out in various conditions on actual roads over 1.5 to 2 hours over a distance of around 50 miles. RDE ensures real-world conformity with the WLTP lab testing.

WLTP testing and plug-in hybrids

If you've previously taken an interest in WLTP fuel and energy efficiency figures, you will likely have spotted that many plug-in hybrid (PHEV) models have exceptional WLTP Combined cycle results, often well in excess of 250mpg. So what's going on?

Starting with the battery charged to 100% capacity, the Charge-Depleting Phase permits PHEVs to undertake the test potentially without the engine ever firing into life. The larger its battery pack, the longer its potential electric-only driving range will be, reducing the percentage of the test undertaken with the engine running, possibly down to 0%. 

Recognising that not everyone charges their PHEV battery, the Sustaining Phase replicates the test with the 'plug-in charge' fully depleted. At this point, PHEVs operate similarly to self-charging hybrids (HEVs), with wildly different efficiency results than the Charge-Depleting Phase.

From these different test types, a weighted average is calculated to arrive at the Combined cycle fuel efficiency and CO2 emission figures, but evidently with a greater proportion of that skewing assuming electric-only driving.

Introduced on 1 January 2025, with manufacturers given until the end of 2025 to comply, changes to PHEVs' WLTP Combined cycle calculations – from Euro 6-EA to Euro 6-EB – were designed to better replicate real-world economy such cars should achieve. The downside is that their official CO2 emissions have simultaneously increased, making them less attractive for company car drivers from a Benefit-in-Kind (BiK) perspective.

Quirks with WLTP testing figures

For petrol, diesel, mild and self-charging hybrid cars, WLTP Combined cycle fuel efficiency results are published in mpg (miles per gallon), with no obligation for manufacturers to publish a driving range figure based on them. That can be calculated by multiplying the mpg figure by the fuel tank capacity.

That's different from electric cars where the requirement is for the WLTP Combined cycle to be given as an overall driving range figure. Many manufacturers additionally publish WLTP City cycle range figures and quote electrical efficiency results, but not always with the same metrics. UK motoring publications and websites tend to favour mpkWh (miles per kilowatt hour), but you may also encounter a European equivalent of kWh/100km, yet calculating the efficiency by dividing the driving range by the battery capacity tends not to give an accurate mpkWh result.

Plug-in hybrid cars have their electric-only Combined cycle ranges published – but it's less common for an electrical efficiency figure to be quoted – and the weighted Combined cycle fuel efficiency level, made more realistic during 2025. Few manufacturers publish their Sustaining-Phase mpg figures meaning calculating PHEVs' overall driving range potential – Sustaining-Phase mpg multiplied by the fuel tank capacity plus the electric-only range – is rarely possible.

Why should I about care WLTP?

On a fundamental level, WLTP enables car buyers to make even more informed decisions than before when comparing fuel and electrical energy efficiency levels. Yes, they're still a standardised test, which reduce variables to make those comparisons fairer, but because they're closer to real-world efficiency levels it enables consumers to calculate their potential running costs with a greater degree of accuracy than before.

One element of WLTP testing that's not immediately obvious is that they require car manufacturers to submit each individual model derivative for testing, including with optional extras. That's because their combined weight as well as tyre dimensions and their levels of rolling resistance, will all influence both efficiency and emissions levels. 

Each combination submitted for testing has a cost, meaning most manufacturers have significantly reduced both the number of model derivatives available and bundled many optional extras together in packs – if they are still available at all – since WLTP's introduction. 

While this means that there are fewer model permutations to choose from, for those which do remain there's a greater degree of transparancy between derivatives, improving consumer confidence when it comes to making the right choice for them.