Just two cars deliver their advertised fuel economy when on the road, with the thousands of other models 30% worse on average in the real world, according to comprehensive new data.
Some cars, such as the Fiat 500 and Ford Fiesta, gave barely half the mileage advertised.
The result is that drivers are being misled and paying far more to drive, say experts, who warn that a stricter official test coming in 2017 will only close about half the gap between official and real fuel efficiency.
The data from leading testing company Emissions Analytics covers 60,000 models and was published on Thursday, the first such database available to the public. It uses onboard equipment to measure mileage over four hours of real-world driving. In contrast, the official regulatory test is a gentle lab-based exercise.
The worst gap between official miles per gallon (MPG) and real-world performance was for the Fiat 500, which is rated at 70.6MPG, but only delivered 39MPG on the road, a 45% drop. Other popular petrol cars performing at least 40% worse on the road include the UK’s most popular car, the Ford Fiesta, as well as the Ford Focus, Toyota Yaris and Mini Hatch. Some diesels, which generally have better fuel efficiency, also had 40% gaps, such as the VW Golf and Peugeot 308.
The only cars to produce better fuel efficiency on the road were the 4.7-litre engine Aston Martin Vantage, which gave 21.5MPG in the real world, 5% higher than in the lab, and the 3.7l Nissan 370Z, which was 1% better on the road at 26.6MPG.
The best on-the-road mileage was produced by the Honda Civic, which did 61.8MPG in the real world, though this was still 21% lower than its official mileage of 78.5MPG. The Citroen C3 was next best, with 60.3MPG, 28% lower than its official rating. The worst actual fuel economy came from the BMW X5, with just 16.2MPG, the Range Rover Sport (17.5MPG) and the Porsche Cayenne (17.8MPG), all well below official ratings.
From 2017, a new official test comes into force, which is more strenuous on the car’s engine but is still lab-based. “Drivers have been misled by the official numbers, but even when the new system comes in next September, it won’t solve the problem,” said Nick Molden, CEO of Emissions Analytics. “Currently the real-world performance of cars is on average 29% worse than the official test. Our estimate is that this gap will close by about half, meaning a 10-15% gap will still exist.”
“It is still a lab test, with no hills, cornering or operating air conditioning,” he said. “You have to test on real roads to know what cars really do in normal driving.” This happens in the US, where road tests are used to police the system.
Julia Poliscanova, clean vehicles manager at campaign group Transport & Environment, said: “The fuel consumption gap has become a vast chasm. Carmakers’ manipulation of the weak, outdated lab test is widespread, affecting diesel and petrol cars. This means a total waste of motorists’ money and an increase in global warming emissions.”
“National regulators have been turning a blind to this evidence,” she said. “So Europe now needs to cross this chasm and introduce on-road testing. The equipment to do this has been widely available for years, but the political will, as Dieselgate has shown, is sorely lacking.”
Mike Hawes, chief executive of UK trade body, the Society of Motor Manufacturers and Traders (SMMT) said: “The current test for fuel consumption is outdated and a new, more stringent lab test from later next year is welcome.”
“This will be much more representative of on-road driving and provide the necessary repeatability to allow consumers to compare accurately individual model performance,” he said. “No single lab test can ever replicate exactly real-world conditions, given infinite variations in temperature, load, speed, maintenance, gradients, and traffic, but industry will continue to invest in new technologies to deliver ever greater fuel economy.”
Overall, the Emissions Analytics data showed that cars with small engines had the biggest gap between official fuel economy and actual performance, and those with the biggest engines had the smallest gaps.
“The official test cycle is too gentle, so it encouraged the downsizing of engines to go too far,” explained Molden. “A little one litre engine will do very well, often 80MPG, on the official test, but that has no hills, no passengers, no operating air conditioning. So they suffer disproportionately when you put some weight in the boot and take it up a hill – it is basically underpowered.”
“The Aston Martin Vantage is the opposite: it is overpowered,” said Molden. “It does badly in the lab test and on the road it doesn’t do any worse. It has just got more power than you could possibly need for normal driving.”