PHEVs caught in ‘real-world’ fuel consumption conundrum

03 March 2021

A new study by the UK consumer group Which? has said manufacturers are 'advertising' unrealistic fuel efficiency figures for plug-in hybrid electric vehicles (PHEVs). The group discovered the drivetrain was 61% less fuel-efficient on average than the official rating.

‘A fuel-efficient plug-in hybrid vehicle is an attractive feature for prospective buyers, as many will expect to spend less on fuel and reduce their carbon footprint. Yet our research shows many hybrid models are not as efficient as the manufacturers claim, which means motorists could be spending more on fuel than they anticipated,’ said Natalie Hitchins, head of home products and services at Which?

‘It is clear that the standard set for calculating fuel consumption is flawed and should be reviewed to better reflect real-life driving conditions. This would ensure manufacturers advertise more accurate figures and consumers have a better understanding of how much they should expect to spend on fuel.’

Carmakers and industry bodies leapt to the defence of the technology. They pointed to the essential nature of PHEVs as a stepping-stone, bridging the gap between internal combustion engines (ICEs) and battery-electric vehicles (BEVs). As the drivetrain continues to be the subject of examination by third parties, the sector also identified the importance of standardised testing, as a vehicle’s performance is accountable to a great many variables.

Official vs real-world

Which? subjected 22 popular PHEVs to lab tests that were ‘tougher than official tests to better represent real-driving conditions.’ Overall, the consumer group found that models were 61% less fuel-efficient on average than in official fuel ratings. These outcomes were then applied to average fuel costs and an annual mileage of 9,000 to predict how much fuel would cost every year for each car.

BMW’s X5 PHEV was found to be 72% less efficient. The consumer group estimated this could cost the owner £669 (€775) more a year in fuel. According to the carmaker, the X5 could achieve 188.3 mpg on the Worldwide Harmonised Light Vehicle Test Procedure (WLTP). But the model could only record 52.8 mpg under Which? testing conditions.

Mercedes’ B-Class PHEV was 67% less efficient than claimed. According to the carmaker, it could reach 256 mpg, meaning customers might expect to spend £206 a year on petrol or diesel. But in Which?’s tests, it reached 78mpg, equalling an annual fuel bill of £617.

The Toyota Prius was found to be 39 % less fuel-efficient than in official figures. The OEM claims it can achieve 188.3 mpg, however Which? tests found it was 114.4. Based on Toyota’s mpg, Prius owners might only expect to spend £257.62 a year on fuel, but using Which?’s calculations, consumers would likely spend £429.07 a year.

Essential stepping stone

‘Fuel consumption published for PHEVs needs to comply with the same rules as any other powertrain technology. The challenge around the PHEV technology is that if drivers do not charge it diligently before every trip, fuel consumption rises above comparable ICE vehicles,’ said Autovista Group's chief economist Christof Engelskirchen.

‘Fleet operators are beginning to put proper-use-of-PHEV-technology-policies in place to encourage drivers to charge. Depending on the driving pattern, fuel consumption can then be below those official values.’

Put to the test

‘The legally required WLTP test is designed by the international regulators to be a standardised method of comparing vehicle efficiency, thereby enabling direct comparison between different cars and different technologies,’ BMW responded to the Which? report. ‘These tests show clearly that PHEV technology, when the vehicles are charged regularly as intended, can save significant fuel consumption and emissions over the equivalent petrol or diesel models.’

This links back to the importance of PHEVs as a tool to get consumers accustomed to electric driving. According to the spokesperson, millions of drivers already have the chance to cover substantial parts of their daily commute with their PHEV’s electric drive. In the meantime, the ICE component is available for longer drives when needed. ‘The BMW X5 plug-in hybrid, for example, can travel up to 54 miles in electric-only mode. With technology evolving and expanding charging infrastructure, the customer benefits of PHEV technology will continue to grow,’ BMW concluded.

Mercedes said that it would not comment on the results of the study without understanding the methodology of the testing, owing to the many factors that can affect efficiency. ‘Official WLTP testing is performed in repeatable conditions and certified by government agencies – so Mercedes-Benz customers can be assured they provide accurate and comparable results. We want our customers to benefit from and to use the full technical potential of our plug-in hybrids,’ the company told Which?. ‘We would encourage customers to charge the battery on a regular basis, for example. We also support our customers with getting the best out of their plug-in hybrid – for example, with our Mercedes me connected services and elements such as our ECO Coach.’

Toyota also raised a flag on the methodology compared to the current accepted testing system. ‘Fuel efficiency statistics are the result of mandated, WLTP homologation procedures,’ they said. ‘Without knowing the testing regime undertaken in this particular study, it is impossible for car manufacturers to draw comparisons.’

The company added that efficiency and emissions depend upon usage. ‘There is a plan for ICE, HEV and PHEV vehicles registered across the EU to be fitted with an on-board fuel and energy consumption-monitoring device to verify the real-world consumption of vehicles, as well as the usage patterns for PHEVs overall. This will give a true picture of the situation regarding the use and emissions of PHEVs. The European Commission will use this data to re-assess the real-world usage of PHEVs and, potentially, make the adequate legislative adaptations,’ it said. ‘Under the right conditions, plug-in hybrid vehicles offer an excellent transition to zero-emission mobility.’

Required by law

‘By law, manufacturers are required to test all vehicles of all technologies to the same, repeatable standard – the WLTP Test, which is independently verified by government authorities and it is these results – and only these results – that manufacturers are required by law to publish within any advertising communications,’ commented Mike Hawes, chief executive of the UK Society of Motor Manufacturers and Traders (SMMT).

He went on to say that there will always be a difference between lab tests and real-world use cases. Fuel usage is subject to many different variables, such as the type of journey, road conditions, driving style, and vehicle load. As a standardised measuring stick, the WLTP test is designed to work around these variables so consumers can get accurate and comparable results.

‘The WLTP tests consistently demonstrate that PHEVs offer comparable range to pure petrol or diesel equivalents but deliver substantial emission reductions, with zero-emission range typically 25-40 miles, which is more than ample given that 94% of UK car journeys are less than 25 miles. PHEV range and performance will continue to improve, meaning that, for many drivers, they are the essential stepping-stone to a fully-electric vehicle,’ Hawes concluded.

Continuing global criticism

Beyond the UK, Transport and Environment (T&E) also conducted a ‘real-world’ study of PHEV emissions, publishing the results towards the end of last year. They found that even under the mildest conditions with a full battery, the cars’ emissions were higher than advertised.

Another report published by the International Council on Clean Transportation (ICCT) in September last year reviewed real-world consumption of roughly 10,000 PHEVs in Europe, China and North America. They found that fuel consumption and CO2 emissions were about two to four times higher on average than the New European Drive Cycle (NEDC) type-approval values. 

The ICCT also found that, on average, PHEVs spend roughly half the amount of time driving electric that is considered in the type-approval values. For private cars, the average utility factor (UF) - the portion of kilometres driven electrically vs ICE - is 69% under type-approval but only 37% in the real world. For company cars, there is an average UF of 63% NEDC vs 20% for real-world driving.

However, this does point towards driver behaviour as a massively decisive factor in the employment of the electric drivetrain. Even if real-world conditions do not perfectly mirror lab-based results, driver treatment will ultimately decide the efficiency of the vehicle. If it is far easier to fill up with petrol, rather than finding a plug-in point, there is likely to be a sizeable portion of consumers who will do that.

This is something fleet managers could tackle in the recharging rates of company cars. If recharging and utilisation of the electric drive is made mandatory within the company policy, the PHEV stands a better chance of meeting its type-approval values. Equally, with the introduction of more charging infrastructure and greater incentives, private PHEVs could also see more efficient usage.

Monumental growth

Last year, PHEVs saw a monumental growth in registrations across the European Union, up 262.3% compared to 2019. As acknowledged by the European Automobile Manufacturers Association (ACEA), this is likely thanks to green incentives spurring the adoption of low-emission vehicles. PHEVs currently hold their (absolute) RVs better than their peers in most markets, but this is partly driven by them being high-performance variants that attract buyers. They are beginning to come under pressure though, owing to more supply and the high government incentives for new PHEVs.

RV performance by fuel type in Germany after 36 months and 60km

RVI Germany

Source: Autovista Group, Residual Value Intelligence

The market for PHEVs is undoubtedly growing, and it will be important to find markets that will absorb the used cars that will come in higher volumes over the coming years. So, while ICE models are phased out by manufactures looking to meet emission targets, and BEVs find their feet as a developing drivetrain, PHEVs will continue to act as a stepping stone. With one foot planted in the known world of fuel pumps, and the other exploring the possibilities of electric plugs, the true value of PHEVs sits firmly as a bridge between the two worlds.