Are plug-in hybrid automobiles pulling the eco wool over our eyes? Study claims they guzzle THREE TIMES extra gasoline than marketed

Plug-in hybrid vehicles (PHEVs) are far less economical than motorists are being led to believe, a damning new report has found.

The electrified vehicles, which are able to run on electric power for short periods before a petrol engine kicks in and are labelled as a ‘steppingstone’ to fully electric cars, have had their eco credentials put under the microscope by German research organisation, the Fraunhofer Institute.

In what what has been dubbed ‘the most comprehensive study of its kind to date’ using analysis of around a million PHEV vehicles, it discovered that they are guzzle far more fuel than advertised.

Instead of using one to two litres of fuel per 100km, but they need six litres on average, it concluded. 

To ascertain these figures, the institute used data transmitted wirelessly by PHEVs which allowed analysts to determine precise, real-world fuel consumption for cars produced between 2021 and 2023.

These figures differ hugely from the numbers stated in the vehicles’ official EU approved certifications, with the study suggesting that real-world CO2 emissions were also three to five times higher than manufacturers’ advertise. 

Despite only being just over half way through February, this is the second report of its kind blasting the eco credentials of PHEVs, which have already been described as a ‘scandal with echoes of Dieselgate’.

How PHEVs used more fuel consumption than stated

Fraunhofer Institute found real-world fuel consumption is 5.9 litres/100km – or 48mpg – which is around 300 per cent above the type-approval consumption. 

In fact, PHEVs show fuel consumption on the road equivalent to conventional internal combustion cars. 

Vehicle makers had claimed that plug-in hybrids use hardly any fuel when in electric mode, but the study has shown that this might not be the case. 

Patrick Plötz, who carried out the study, said in an interview with German broadcaster SWR that the higher-than-stated fuel usage is due to PHEVs switching between modes – the electric motors and the combustion engine.

And he believes the combustion engine is kicked into action more than previously believed.

The study shows that the petrol engine can be engaged much more frequently in ‘discharge mode’ (when charge is depleting) than previously stated, and as such the consumption engine plays a significant role in this mode.

Which models use more or less fuel than thought?

It turns out that many cheaper PHEVs from brands like Kia, Toyota, Ford and Renault have the lowest fuel consumption.

These plug-in hybrids typically used under one litre per 100km – the equivalent to a staggering 282mpg.

On the other hand, Porsche hybrids consume far more fuel – around seven litres per 100km (roughly 40mpg). 

This can be up to 85 per cent more fuel than cheaper PHEVs.

A Ford Kuga hybrid can cost from just over £33,000 while a Porsche Cayenne E-Hybrid is over £86,000.

Porsche told SWR that the discrepancy is due to ‘differing usage patterns’ and that the fuel consumption measures were in line with EU legal requirements.

Colin Walker, Head of transport at the Energy and Climate Intelligence Unit (ECIU) think tank said the study is ‘further evidence that PHEVs are not delivering what is promised of them’. 

He told Daily Mail and This is Money: ‘Despite this latest report finding that they consume 300 per cent more fuel than their manufacturers claim, they are being misleadingly marketed as cars that will help save people money while also cutting emissions. 

‘This chimes with our own analysis showing that hybrids actually cost more to own and run than a regular petrol car.’

Calls for updated PHEV regulations

Having reflected on its findings, the institute has called on EU regulators to adjust their measurements for PHEVs during official tests.

There are already existing proposals that PHEV users should be shown the proportion of electric driving transparently on the display (‘display transparency’) or be forced to charge at least every 500km (311 miles) to reduce their emissions impact.

In January 2025, the European Union introduced the Euro 6e-bis emissions standard, a new regulation that brings stricter testing procedures for plug-in hybrid electric vehicles (PHEVs). 

It aims to close the gap between lab-based emissions results and actual real-world driving conditions, ensuring more accurate reporting of CO₂ emissions.

While the UK is no longer in the EU, it has historically aligned with EU vehicle emissions regulations, and similar rules still apply to vehicles sold and registered in the UK post-Brexit.

The recent fuel bill scandal – how much more do PHEVs cost to run than claimed? 

In January, This is Money reported how that owners of PHEVs are ‘likely spending almost twice as much money to fuel their cars than their manufacturers have led them to believe’.

The ECIU called it ‘a scandal with echoes of dieselgate’.  

Motorists are told their annual fuel and electricity bills per annum for the most popular PHEVs sold in Britain will amount to around £530, but data showed that the reality is more like £985 a year.

This is because ‘petrol is more expensive per mile than electricity’ and PHEV owners typically don’t charge their cars regularly enough to make the most of the electric benefits, it’s claimed.

A spokesman for the Society of Motor Manufacturers and Traders retaliated to last month’s findings, telling the Daily Mail and This is Money that car firms ‘don’t make ‘claims’ about fuel efficiency’.

They told us: ‘By law, they [car makers] are required to test all vehicles to the same repeatable, government-verified standard – the WLTP Test – and by law they must publish the results in all advertising.

‘Industry works with international regulators to evolve the WLTP test as more data becomes available.

‘With some still concerned about the availability and cost of public charging, manufacturers have developed a range of technologies so everyone can reduce their emissions.

‘Plug-in hybrids can be a crucial stepping stone to going fully electric, allowing people to experience the benefits of zero emission driving and helping to overcome anxiety.’

Daly Mail and This is Money has also contacted Porsche for comment.