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.
