CAR tech: who's to blame for your car's terrible fuel economy?

Published: 12 August 2013

In early 2013 Audi lost a case brought by the Advertising Standard Agency (ASA) because of ‘misleading’ fuel economy figures used in an advert, after a customer complained they couldn’t get anywhere near the 68mpg quoted.

The court case once more exposes the yawning gap between officially sanctioned mpg figures and those experienced by owners. A recent study by the Independent Council on Clean Transportation (ICCT) looked at cars sold in the UK and Europe, and discovered the difference between official mpg figures and real-world driving had grown from 8% in 2001 to a barely believable 21% in 2011. And the gap is getting wider.

Every car sold today is less polluting and more fuel efficient than the one it replaces, with average CO2 emissions for UK cars dropping from 177g/km in 2001 to 133g/km in 2012. Commendable, but it doesn’t solve the problem of the gap between official figures and the real world. What is going on?

The problem isn’t with the cars; they really are getting more fuel efficient and they certainly do pump out fewer harmful pollutants. And it’s not to do with the distance we’re travelling; we are driving fewer miles than we have for a long time, as the world’s economic woes conspire with the rising cost of fuel to dampen car use. So is the problem solely with the way the official manufacturer testing procedure is carried out?

There appear to be several reasons for the yawning gap. The procedure is partly to blame, by setting hugely optimistic mpg figures because of the artificial way the test is carried out. The ICCT claims 40% of the difference is down to the way we drive, especially on faster roads, which bears no relation to the New European Driving Cycle (NEDC) test.

It seems the remaining 60% of the disparity is down to the rising number of turbodiesels on the road, as they are particularly susceptible to the differences between the testing and real-world driving.
The finger can be pointed at car companies too, who know precisely how to make cars excel in the lab. Overinflating tyres, taping over panel gaps and grilles, and using special lubes are among the sneaky ways to maximise results. The rise of the smooth twin-clutch gearbox and changes to gearing help too.

Inside the MPG torture labs:

Today's test: New European Driving Cycle Proposed test: Worldwide Harmonised Light Vehciles Test Procedures
Total distance covered An exhaustive 6.8 miles A still low 14.3 miles
Test location A laboratory roling road heated to betwen 20-30 degrees Centigrade Still in the lab. Not clear if the place is heated
Maximum speed reached 75mph –  which is lower than the French motorway speed limit A more realistic 81.4mph
Duration of test A hellishly long 19.6 minutes An endurance testing 34 minutes
CAR's verdict A benchmarking test which appears to benchmark nothing related to reality. What is the point, we ask? Certainly an improvement over the current sham but still no replication of the sorts of real-life driving we all do


One insider told CAR: 'NEDC is a repeatable test to allow cars to be compared to one another - nothing more, nothing less. It isn’t a predictor of real-world driving performance and has never claimed to be. 'The NEDC cycle consists of a fairly lame set of tootling about most people would struggle to replicate, it’s so gentle. The point is that pretty much any vehicle is capable of performing it; it is specified so that even the feeblest vehicle can complete it, and then some. There is a little bit at the end where the car gets up to highway speed, but it doesn’t last long.
‘Modern turbodiesels are driving off-boost for a lot of the NEDC and they are very economical at those revs. Non-turbodiesels gave ridiculous fuel economy but don’t exist any longer. We have more and more diesel cars in the fleet and that alone explains the growing difference between NEDC and so-called “real world” fuel economy.'

In Europe 48% of cars sold are now diesel, up from just over 20% a decade ago and this skews the figures. Bigger, naturally aspirated, petrol engine cars return figures much closer to the official mpg. Those cars worst affected are often those touted as the ones with the biggest eco credentials, as they do so well in testing but once out on motorways and faster roads the mpg figures tumble.

Ford spokesman Oliver Rowe said: ‘There are two elements to this; the car, and the environment. Only the car can be tested in the same way, as all the other variables constantly change. All of those variables from the weather, road surface, using the air conditioning or heated windows can halve the tested combined figures.

‘Achieving the combined figure is never a given but we have seen in fuel economy competitions where a Fiesta Econetic can achieve 108mpg against a combined of 85mpg. But that is not generally how people drive.’

Changes are on the way but they aren’t a complete fix. It seems there may be no completely accurate way of replicating fuel consumption in the lab, because to remain consistent the test has to be artificial.

A new Worldwide harmonised Light vehicles Test Procedure (WLTP) is a partial answer. It will increase the length of the test, be divided into three grades for different power-to-weight ratios, and reach higher speeds. But in spite of political and consumer group pressure, it’s unlikely to be implemented for three to four years at the earliest. In the meantime, you’ll have to mind the gap, when it comes to a buying decision based on a car’s fuel consumption.

When it comes to who’s to blame for the fuel economy gap, the answer is everyone: foot-dragging politicians, sneaky car companies and toothless testers. And all of us drivers. We have the power to improve fuel economy in our own hands. Or feet. We just have to go easier on the pedals, worse luck.