Plug-in hybrids not as fuel-efficient as official figures claim, Which? reveals

Manufacturers are advertising unrealistic fuel efficiency figures for plug-in hybrid cars and potentially costing motorists hundreds of pounds more in fuel costs, according to new tests by Which?.

The consumer champion tested 22 popular models and found they all fell well short of car makers’ claims, with the worst being 72 per cent less efficient than claimed and the ‘best’ still being 39 per cent below its official fuel economy rating.

Across all of the tests – which are tougher than official tests to better represent real-driving conditions – Which? found the cars were on average 61 per cent less fuel-efficient than the official fuel economy rating, meaning motorists could be paying on average £462 more a year in fuel costs.

Fuel consumption figures are an important factor for consumers when buying a plug-in hybrid vehicle. However, manufacturers’ fuel consumption figures – miles per gallon (mpg) – are calculated using the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) and are based on a model’s electric driving range, among other factors. This means the mpg that can actually be achieved in real-world driving can be much lower and drivers could consume and spend more fuel than expected.

In one particularly bad case, the consumer champion found the BMW’s X5 plug-in hybrid was 72 per cent less efficient than official figures and could cost the owner £669 more a year in fuel costs. According to BMW, this model could cover 188.3 miles per gallon, but Which? tests found it could only cover 52.8 miles per gallon. Which? calculated, based on the manufacturer’s mpg, owners might expect to spend £258 on fuel annually, however, using Which?’s mpg figure, they would spend £927 a year.

BMW’s 2 Series Active Tourer was also 71 per cent less efficient than the manufacturer claimed. BMW claimed this model could cover 156.9 miles per gallon, but Which? tests found it could only cover 44.8 miles per gallon and would cost £1,081 a year to fuel. This is compared to £309 a year based on the manufacturer’s mpg – a £772.08 difference.

Which? also found Mercedes’ B-Class plug-in hybrid was 67 per cent less efficient than claimed. According to Mercedes, it could cover 256 miles per gallon, which means customers might only expect to spend £206 a year in petrol or diesel costs. But in Which?’s tests, it could only cover 78 miles per gallon, meaning fuel costs would be much higher than anticipated at £617 a year.

While the Toyota Prius was “best” among all the models tested, it was still 39 per cent less fuel efficient than official figures claim. Toyota claimed the mpg for its Prius model is 188.3 miles, however Which? tests found it was 114.4. Based on Toyota’s mpg, Prius owners might only expect to spend £257.62 annually on fuel, however using Which?’s mpg would likely spend £429.07 a year.

Natalie Hitchins, Head of Home Products and Services at Which?, said:

“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.

“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.”

Notes to editor

Which? tested 22 plug-in hybrid vehicles in rigorous and independent lab tests, designed to replicate real-world driving conditions, and used its actual fuel consumption over a fixed distance (100km for comparability purposes) to calculate its MPG.

Using this fuel consumption figure, average fuel costs (118.5p for petrol and 121.8p for diesel) and annual mileage of 9,000 (average travelled by respondents in Which? ‘s annual car survey), Which? calculated how much petrol or diesel would cost every year for each car. It compared this with annual fuel costs based on the manufacturer’s lowest average mpg claim.

More information on how Which? tests MPG is available here:

Please see a link below to cars tested (figures used in the table are based on the model specifications available at the time of testing):

Rights of reply

A BMW spokesperson said:
“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. 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.

“Plug-in hybrid technology is important to get customers used to electric driving and demonstrate how practical electrified driving is in every-day life. For millions of drivers, today’s PHEV technology already offers the opportunity to cover substantial parts of the daily commute, if not all of it, using only electric power, whilst having the flexibility of the combustion engine available for longer drives when required. 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.”

A Mercedes spokesperson said:
“We cannot comment on your results without understanding the methodology of your test – given the many factors and scenarios that can affect mpg. 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.

“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.”

A Toyota spokesperson said:
“Fuel efficiency statistics are the result of mandated, WLTP homologation procedures. Without knowing the testing regime undertaken in this particular study, it is impossible for car manufacturers to draw comparisons.

“Equally, the efficiency and CO2 emissions of PHEVs depend on how they are being used. 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.

“Under the right conditions, plug-in hybrid vehicles offer an excellent transition to zero-emission mobility.”

A spokesperson from the Vehicle Certification Agency said:
“The fuel consumption of plug-in hybrid electric vehicles (PHEVs) is determined through rigorous lab testing procedures.

“Fuel consumption in real world driving conditions is influenced by many factors and for PHEVs this includes the ability to switch between electrical and combustion engine modes. Without detailed knowledge of the test vehicles, drive cycles, methods of determining fuel consumption plus a number of other factors, it is not possible to comment upon efficiency claims from independent testing.”

How to get the best fuel economy from a PHEV

  • A PHEV will cut your running costs, compared to a traditional combustion engine, if you regularly drive in town within the battery’s range.
  • This means you need to keep it charged, otherwise the combustion engine will kick in – and not only will it have to power your car, but it’s also having to carry the weight of a heavy hybrid system.
  • If you can easily keep it charged, consider an electric car as it will likely be cheaper and cleaner to run overall.

The information in this press release is for editorial use by journalists and media outlets only. Any business seeking to reproduce information in this release should contact the Which? Endorsement Scheme team at

Which? is the UK’s consumer champion, here to make life simpler, fairer and safer for everyone. Our research gets to the heart of consumer issues, our advice is impartial, and our rigorous product tests lead to expert recommendations. We’re the independent consumer voice that influences politicians and lawmakers, investigates, holds businesses to account and makes change happen. As an organisation we’re not for profit and all for making consumers more powerful.

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