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Official MPG – fact or fantasy?
How is fuel consumption calculated and why do most of us never get close to the 'official' consumption figure we are offered?
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.
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. F
irst 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.
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 practise 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 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.
The Government fuel consumption figures are just rubbish. They can never be accurate as there are so many variables. But they do help the government to lift more hand earned out of us.
The only fair way to tax a vehicle is through the fuel system. Drive a lot . . . pay a lot and the reverse.
albert hall, hove, england
My large, heavy, BMW 5 series saloon, whilst sometimes perceived as a gas-guzzler - actually does 45+mpg and with its particulate filter produces no smoke. I would challenge any Prius driver to beat that on a daily basis. Plus it can do 140mph on an autobahn, carry 4 plus luggage in comfort and doesn't have the problems of disposing of all those batteries when it needs to be scrapped. Current hyrbid technology will be proved over time to be nothing more than another passing fashion - IMO of course! And by the way, if you think it's too expensive a 118d is well under £20k :-)
Matt L, Chichester,
Would a diesel hybrid not be the best of both worlds?
Ads, Nottingham,
my Kia Rio 1500 diesel bought in 2005 only managed 49.6 mpg in a recent test by myself. I returned with the car to the garage i bought it from (done under 1000 mls) and got them to check fuel consumption, of course they stuck it on there little computer and low and behold they said all is ok. right next it was a 30 mile road run at 60 mph, fine before leaving they told me they would be expecting a mile or two either side of the advertised 60 mph and if outside this figure they would pay for the fuel used. well it managed 55 mpg, they then got in touch with Kia head office who said because the wind was a bit gusty it's an acceptible figure. ( they did'nt pay for fuel either). To cut a long story short i would advise any person "DO NOT BUY A KIA RIO". Kia did say it would improve when it has done 10000 miles, its now done about 17000 and as i said at the start its doing 49.6mpg. ARRRRGH
richard howe, norwich, norfolk
I only get 45 mpg from my Kia Rio diesel 2007 model-- not the 60mpg as advertised. Anyone out there getting a better average? If not why don't we form an action group-- whoever
quotes the "official government figures" isn't quoting for my car.
I bought the car BECAUSE it would do 60mpg (supposedly-- what rubbish..) I am 150 miles down on a tank of diesel, going by the "official government figures".
Merryle Griffiths, Blackwood Gwent,
I drive a Peugeot 206 1.1 petrol and i am lucky to achieve 33mpg driving in town although I managed 50mpg on a run to Dorset it cost me £12.50 for 140miles. Has anyone tried those fuel additives which are meant to improve MPG? Just wondered if it would help my car as i feel 33mpg Urban isnt great form a 1.1.
Will, Portsmouth, England
I get 18mpg from my BMW 540i on avergae and I don't give a hoot because its only 8 miles to work and I can still afford to burn off everything in sight. Live it up badboys!
Jock, Dundee, Scotland land of the Free
Brian in Farnham, UK is the person who gets the Golden Star Award for getting the US to Imperial gallon conversion wrong. And for advising poor old Adrian Ryan he got it wrong when he in fact got it right the first time
It's simple. Forget about fluid oz and all that stuff. The US and Imperial fluid ozs are different anyway. All one needs to know is that there are 3.788 litres in a US gallon, 4.545 litres in an Imperial gallon. That's a ratio of 0.8334. Near enough to be exactly 5/6 ths. 58.85 mpg Imperial is 49.04 mpg all right. So, Mr. Ryan, you need not hang your head in shame as an engineer! Other than why did you agree the other fellow was correct and eat crow? He's wrong, pure and simple. We get this problem all the time here in Canada, as we used to use the Imperial gallon before going metric, and the slumbering elephant of the US next door and it's major influence on our economy meant that we have to check their fluid measurements all the time.
Bruce Armstrong, Halifax, Canada
The guys down in Swindon at Honda UK told me that keeping the speed constant is the key to good fuel economy.
Andrew Milner, Yokohama, Nagano
A lot of well argued points - but there is a much simpler reason.
CO2 is produced by burning fuel, the more petrol burned the more CO2 produced -simple! So if you multiply the CO2 figures for a range of vehicles by the fule consumption figures you will get a constant figure.
WRONG: try it and see. It these 'official' figures can't even stand up to a simple mathematical test then its fairly obvious that some bureaucratic imagination got involved!
MIke Bibby, St Albans, England -not EU
You are right, Brian, I hang my head in shame, as an engineer, I should have double checked - 58.85 mpg Impereial is indeed 44.1 mpg US, my aplogies to eveyone. I also agree with your comments re motorcycles, and were it not for my wife hating these things, I would still have one. I purchased my Prius not because of its so-called environmental creds, but to explore the use of this technology. Ultimately I think that we will all eventually use electric vehicles and that hybrids will ultimately be seen as an interesting technological diversion. As an engineer, I have to say that technologically, the Prius is rather akin to playing the trumpet by sucking through the large end!
Adrian Ryan, Donegal, Ireland
Killjoys!! Life is for living, if you don't give two hoots about mpg figures but are rather looking to have some fun then most of the comments here are not worth reading. I do believe that large swathes of the UK/World population agree with me. Make no mistake i do not underestimate the importance of being frugal with the throttle and brakes when driving, for many obvious reasons i.e. anticipating other drivers behaviour, giving oneself ample time to react to obstacles on the road etc.
ANDREW, HACKNEY, LONDON
My last two cars have provided me with exactly the manufacturer's figures. 36 mpg (2L petrol) and 47 mpg (2L diesel). On a long trip at a steady 70mph I once recorded 62 mpg in the diesel.
The car I am about to buy quotes 64.2 mpg (1.6L diesel), I have little doubt that I will get very close to that figure.
The main factor in fuel consumption is how hard you press the TWO pedals with your right foot !
Before anyone says it - you won't find many queues behind me on the road either !
John Newbury, Calne, UK
Actually, I have managed to achieve the manufacturer's claims for most of the vehicles I have driven over the last decade or so. My VW Polo was right on the numbers, as was an Opel Corsa, Toyota Corrolla, Jaguar 'S'-Type, and Volvo V40. The only car with which I have difficulty meeting the specs is my present Toyota Prius, but even here, I'm getting close, at 4.8 litres/100 Km, or 58.85 mpg (UK), 49 mpg (US). These figures are averaged over a tankful of fuel, as were the figures for all my other cars, so I'm quite happy.
Adrian Ryan, Donegal, Ireland
Adrian needs to check his maths - 58.85 mpg imperial is actually 44.14 mpg US. There are 16 fl oz in a US gallon - a pint of water weighs a pound over there, not a pound and a quarter. While his figures sound good, I can achieve 40 mpg from my 10 year old BMW 528 using his technique, but it is impossible to drive like that all the time, and frustrates the heck out of other drivers. Personally I prefer my 1200cc motorcycle, which is quoted as potentially getting 60 mpg US, but actually is closer to 50 - and that's over 60 imperial. Not only that, no congestion charge, no parking costs and I can nip through the traffic easily or cruise the autobahns at 100 mph+. Oh, it also has a catalytic converter and puts out very little CO2, and all for less than half the cost of his Prius. Insurance and road tax are cheap, too, at £128 fully comp and £64 respectively!
Brian, Farnham, UK
I think the comments about the Prius are slightly unfair. Whilst I would agree that one can equal the fuel consumption figures in almost any modern 1.4 to 1.6 litre diesel, the comment that it is not suited to motorways is untrue. As evidence, I drove from County Donegal to Hastings in Sussex a distance of 830 kms on a single tank of fuel at an average consumption of 4.8 litres/100km, or 58.85 mpg (Imperial), 49.04 mpg (US), and this mostly on motorways in the UK at normal motorway speeds. I would certainly agree that to do this requires an entirely different driving technique, but this can be summed up as anticipation, very sparing use of brakes, anticipation, anticipation, and anticipation - read the road several vehicles ahead!
Adrian Ryan, Donegal, Ireland
Surely the essence of these figures is comparison. The quoted figures are not what one can expect to obtain on day-to-day driving but they do allow one to make an informed choice. It seems to me to be eminently sensible and has demanded that manufacturers can no longer spew out words like 'economical' in adverts regardless of actual petrol consumption. The incredibly low fuel comsumption of some diesel super-minis is down to these standard figures. How long before someone hits the 100mpg figure?
Derek Smith, Brighton, UK
The main reason that most drivers do not get anything close to the officially quoted MPG for their vehicle is due to poor driving ability i.e. lots of sharp braking, hard accelerating and speeding (obviously).
I've never had much problem getting close to or exceeding the quoted MPG values for any vehicle I've owned and have averaged over 57MPG in my current car simply by driving the way I was taught i.e. "drive smoothly but make progress".
The auto manufacturer's are rightly quoting the capabilities of their product but can hardly be held to account because of their customers' abuse of the product.
Perhaps the advertising for cars should state that "if you drive like an idiot don't expect to get close to the quoted MPG".
Rob Hatcher, Northampton, UK