Are the official MPG figures fact or fantasy?

Did you ever wonder, just how is fuel consumption calculated? And why doesn’t my car ever reach those figures? Our friend Tim Shallcross, from the Car Clinic, explains why*

Since the late seventies, manufacturers have had to tell us the fuel consumption for their products. This requirement was initially prompted by the oil crisis of 1973, then by concerns over the environment. However, fuel consumption varies considerably according to speed and so on, so in order to give a like for like comparison, various standard tests have been introduced over the years. One of the first was the fuel consumption at a constant 56 mph – a strange and rather uninformative figure, since we tend to drive well below that speed in towns and somewhat above it on motorways. The latest standard test was introduced in 2001 and is the same right across the EU. It claims to be more representative of real driving conditions than previous tests; that may be true, but being better than something that’s wildly inaccurate still doesn’t necessarily make it accurate. But how, exactly, does it work?

The testing isn’t done by denim clad curly haired journalists pushing the car to the limit out on the open road; it’s done by very serious folks wearing white coats in a laboratory. They drive the car on a rolling road – the car’s wheels sit on rollers that simulate the resistance of an actual road. There are two parts to the test. The first starts with a cold engine and is supposed to replicate a drive through a town with a specific series of starts and stops and steady speeds for two a half miles. The maximum speed is 31 mph. The second part is intended to replicate out of town driving with speeds up to 75 mph (presumably to allow for the fact that it’s EU wide.) The results for the two parts are quoted, together with a combined figure which is the average of the two. This test is also used to calculate the CO2 figure which is used as the basis for Vehicle Excise Duty (Road Tax) and company car tax. The three mpg figures and the CO2 figure must all be shown clearly in any printed advertisements – and the adverts often promote the CO2 figure in particular, to appeal to company car drivers and to private customers wanting to “go green” and get into a lower road tax band to save some money.

Even the Vehicle Certification Agency (VCA) admits that the fuel consumption achieved on the road will not necessarily be the same as the official test results

However, even the agency responsible for the figures, the Vehicle Certification Agency (VCA) admits that “…the fuel consumption achieved on the road will not necessarily be the same as the official test results”. If the fuel consumption won’t be achieved on the road, nor will the low CO2 emissions. So, why don’t we all manage to achieve the fuel consumption shown on the ads? There are many reasons.

First of all, a rolling road doesn’t really simulate real world conditions. Wind resistance can be simulated to an extent, by extra resistance from the rollers, but this doesn’t include the effect of an open window or a roof box, and they can make a big difference. A roof box can increase fuel consumption by as much as 20 % at 70 mph. As drivers, we often have extra weight in the car. That could be a passenger or the shopping or a set of golf clubs. Weight makes quite a difference when you’re starting and stopping, but for the official mpg test, the car stays still and the load is simulated by the rollers, which means that the impact of any extra weight isn’t picked up during the testing. That can make a difference when there are several variants of a model. Not every car has a sample tested – just one car that is representative of the range – usually just a manual and automatic example for each engine type.

The major factor is that the manufacturer generally wants the test to give the best possible figures

Extras fitted to higher spec. versions that increase weight, drag or engine load are not taken into account. Equipment such as air conditioning, heated rear windows and even headlights use significant amounts of fuel, but are generally (although in fairness, not always) ignored. But the major factor is that the manufacturer generally wants the test to give the best possible figures, and it’s possible to set the car up to perform at its most frugal for the conditions of the test and squeeze into a lower VED bracket, even if those ratios don’t give the best fuel efficiency for real driving. For example, selecting the ideal gear ratios for the conditions of the test cycles may give an excellent set of figures for the advert, but can result in a car that’s awkward to drive at normal speeds – how often do you read reviews of cars that are “boomy” at motorway speeds? Or a car might be set up to do really well on the “out of town” section to give an impressive overall figure, but if it’s driven for much of its life in inner city traffic jams, you won’t be getting the advantages designed into it.

Finally, the test is conducted by a technician who is concentrating on getting the best figures, and knows how to do it. Accelerating reasonably, changing gear as early as possible and minimising the use of the brakes are all techniques we could out into practice out on the road, but few of us do.

Now, all this might seem academic and the figures certainly do have a useful function in that they give good method of comparison between models, since all are doing the same test. They also give you a good target to aim for if you want to drive economically and in an eco-friendly fashion. But they can also be quite misleading.

Take hybrid cars – they use a combination of a petrol engine and electric motor and are generally thought to be very “green”. They’re catching on in the UK now, particularly the Toyota Prius, and they’ve achieved great popularity in the US for a few years now. However, many hybrid owners on both sides of the Atlantic are starting to ask why they’re not getting the miles per gallon suggested by the adverts.

The hybrid concept is very good when you’re crawling along in traffic under 25mph

The reason becomes obvious once you look properly at the figures. For the Prius, the overall fuel consumption, as given by the standard laboratory tests, is a very impressive 65.7 mpg. But owners report that they get nowhere near this – especially those who do quite a high mileage. Why? Well, the overall figure is the average of the “urban” figure of 56.5 mpg and the “extra urban” figure of 67.3 mpg. Both seem impressive until you start to look at other cars – especially small to medium diesel powered cars. Then you see that the urban figure of 56.5 mpg is outstanding and virtually unbeaten by any other car, but the out of town figure is not that impressive by today’s standards. Many manufacturers have similar sized cars that achieve 68, 69 or even 70 mpg or more on the extra urban cycle. That’s because the hybrid concept is very good when you’re crawling along in traffic – when you’re stationary it’s not using any fuel at all and at the pace of a typical urban rush hour crawl it’s running for most of the time on very efficient zero emission electric power. But above about 25 mph at most, the car is running entirely on the petrol engine, at lower efficiency then a good modern diesel. In fact, if the out of town cycle included a long steady speed section, simulating perhaps 20 or 30 miles of motorway, many quite ordinary cars would out perform the hybrids. In town, they’re great; out of town you’re carrying a lot of heavy batteries around for no great benefit. Owners who drive from the suburbs to the suburbs will do very well in a hybrid. Owners who drive from their out of town executive home to the out of town shopping centre or small town high street office would be better off with a modern medium size diesel car – or even one of the very latest “advanced gasoline” engines.

So, don’t believe everything the mpg figures on the adverts tell you – look a little closer and work out the best type of technology for the type of driving you do. And always remember that if you’re not getting the claimed mpg, you’ll also be releasing more climate changing CO2 , so getting the right technology is important if you want your driving to be genuinely eco-friendly.

*reprinted by kind permission of the author, Tim Shallcross, for We Are Futureproof. The article first appeared in the Times Online Driving section. Tim is Head of Technical Advice at IAM