Real World Car Fuel Consumption v. Official Figures

Written on:October 21, 2011
Comments are closed

A BBC radio journalist contacted us recently to ask for our opinion following a complaint from the owner of a Fiat 500 TwinAir. They told us that their listener had recently changed his job to one that necessitated a long commute and he had visited his local Fiat dealer and to ask them for advice. He explaining that his old car returned 40mpg and he wanted to buy a car from them that would give him even lower fuel consumption. They suggested a Fiat 500 TwinAir, quoting its fuel consumption figure of 68.9mpg; he bought one from them.

He was disappointed to discover that his TwinAir was giving him even worse fuel consumption that his old car; he was regularly getting 35mpg and couldn’t understand why when Fiat claim that the 500 will return 68.9mpg as a ‘combined’ figure with up to 76.3mpg on the ‘extra-urban’ cycle. The owner was understandably concerned and returned the car to the Fiat main dealer who sold it to him and suggested that it was faulty. The garage tested the car and could find no fault, something that Fiat UK confirmed. It was at this point that he complained to the BBC and they in turn contacted us and asked for our opinion.

We told the BBC that Fiat’s TwinAir engine is a remarkable feat of engineering. The tiny 875cc, three-cylinder engine produces a very healthy 85PS and just 95g/km CO2, meaning that it has the lowest emissions of any petrol engine anywhere in the world. This is a huge selling point in these eco-conscious days and means that the 500 TwinAir qualifies for free road tax and is exempt from the congestion charge.

However, we then went on to explain that were not surprised that the 500’s owner wasn’t getting anything like the official fuel consumption figures because we didn’t either when we tested one for a week back in August 2011, managing only 40mpg in mixed use. This is something that the 500 TwinAir is notorious for, with the motoring journalist Honest John also reporting that he, and others, struggled to get 40mpg. (Although he did manage to eke out 48.03mpg over two longer journeys.) He even gave it an award, the “Most Brilliant Rule Bender” in May 2011, explaining: “It’s a car that manages to emit so little CO2 in the EC tests that it’s tax and Congestion Charge free, yet as soon as you have any fun in it, its emissions double and its fuel economy is halved.”

So which rules is Honest John referring to and how is the little Fiat so adept at bending them?

The rule in question is actually European Union Directive 93/116/EC (modified by Directive 1999/100/EC), which must also be applied with reference to Directive 91/441/EEC. Simply put, it governs the method by which car manufacturers must assess, measure and report the fuel consumption of their cars. It goes into mind-boggling detail (there is a very good summary here) and the aim to ensure that consumption is measured and reported consistently, allowing consumers and governments to compare vehicles on a ‘like for like’ basis.

The tests are carried out by the car manufacturers themselves, albeit under supervision, and are undertaken on a rolling road in a laboratory. A pre-determined, uniform drill comprising acceleration, braking, coasting and gear-changing is undertaken to replicate town driving over 2.5 miles at an average speed of 12mph (the ‘urban’ cycle) and out of town driving over 4.3 miles and at an average speed of 39mph (the ‘extra-urban’ cycle).

It’s a noble aim made with the best of intentions, but selling cars is a competitive game and the temptation to be able to report that your car is the most economical in its class is just too much. Car manufacturers have twisted, negotiated and interpreted rules since they started making cars (most notably in motorsport) and this is just one more example where the rules can be used to gain an advantage over your competitor; and once one company is doing it they all must follow.

Fiat have been very clever in making a great engine that also performs exceedingly well in the EU fuel consumption tests, allowing them to legitimately make some very bold and impressive claims. But please don’t think that Fiat are being devious or underhand as everyone is doing the same thing, just less successfully…

Some commentators are critical of the current testing regime. Peter De Nayer, a former technical researcher with the AA who now tests real-world fuel consumption for some UK motoring publications, says of them: “Fuel economy in the real world is very complex but the official tests were designed to be simple and repeatable. The results we get from them often don’t indicate the sort of fuel economy motorists can expect or produce a level playing field for comparing different cars. Basically, if you ask a silly question, you get a silly answer.”

Even the UK Government is very clear that the fuel consumption figures published by car manufacturers are unlikely to be attained by owners: “Because of the need to maintain strict comparability of results achieved by the standard tests they cannot be fully representative of real-life driving conditions. Firstly, it is not practicable to test each individual new car; thus only one production car is tested as being representative of the model and may therefore produce a better or worse result than another similar vehicle. Secondly, there are infinite variations in driving styles and in road, car and weather conditions, all of which can have a bearing on the results achieved. For these reasons the consumption achieved on the road will not necessarily accord with the official test results.”

The trouble is that not many motorists understand this and still expect the published figures to reflect the fuel consumption that they can expect to get from the cars that they own.

Where does this leave the consumer and, in particular, our unhappy Fiat 500 TwinAir owner?

A more accurate understanding of a car’s fuel consumption can be gained by using a website that collects and averages ‘real-world’ figures that have been recorded and submitted by car owners. There are a few out there and one of the most respected is that of Honest John. The figure that they give for our Subaru Legacy 3.0 long-term hack, by way of an example, is 24.8mpg. We track its fuel consumption closely using Road Trip and have averaged 25.57mpg over 3,000 miles. If our unhappy Fiat 500 TwinAir owner had consulted the same website he would have found that owners are experiencing an average fuel consumption of 50mpg, better than the 40mpg of his old car but some way short of Fiat’s claim of 68.9mpg.

An alternative method is to subtract 20% from the published ‘extra-urban’ fuel consumption figure, a method that we have found to be very accurate. Our Subaru Legacy has an official extra-urban figure of 31.7mpg but our calculation suggests that 25.36mpg is more likely, which is very close to the 24.8mpg that Honest John reports. Using this method for the Fiat 500 TwinAir though gives a likely fuel consumption of 61.08mpg, far higher than the 50mpg that owners are achieving, suggesting that Fiat’s engineers really do understand what it takes to get a good result in the laboratory…

We told the BBC that, while we thought that the vehicle owner might have been somewhat naive in accepting the published fuel consumption figures at face value, other owners are reporting that their TwinAirs are giving over 50mpg, a figure that should be attainable by most people – if they drive it economically. As their complainant is struggling to get close to 40mpg with his we suggested that he might well be driving it with an eye to performance rather than economy.

We also felt that the dealer should have pointed out that real-world fuel consumption might be lower than Fiat’s figures, especially given the widespread reporting of this engine’s enthusiastic fuel consumption when driven in a spirited fashion, especially given that the customer clearly told them that he needed a car that would better the 40mpg of his old one.

We are pleased to be able to report that the Fiat main dealer offered their customer a refund, something that we thought was very fair as they are under no legal obligation to do. The customer should be pleased with this and Fiat UK should be delighted that they have such a customer-focussed dealer.


Images: Fiat UK


Sorry, the comment form is closed at this time.