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Those Fuel Consumption Figures…

FuelConsumptionLabelI don’t know what you look at first when you think about buying a new car and comb through all the stats in a car review  to see what it’s like. For me, the fuel economy figures would have to be just about top of the list, jostling for space with details like the size of the boot and the number of doors and seats.  However, have you ever noticed that when you actually purchase your new car, you never seem to get the same fuel economy figures that the sticker on the windscreen says?

Sometimes, the reason why you’re not quite wringing the same economy out of your little car is obvious: you may like to accelerate and go fast, you may do heaps of towing, or you may do heaps more stopping and starting and idling than the average driver.  However, even if you’re a light-footed driver who does the average commute, you still might not match the figures in the review or brochure.  So what on earth is going on?  Are you a worse driver than you think you are?  How do they get those fuel consumption figures anyway?

You might imagine that the way the official boffins get the figures is to take the test vehicle and actually drive it around a test track for 100 km at open road speeds, at urban speeds with a few stops to mimic traffic lights and a mixture of both.  That would give a fair impression of what the fuel economy stats actually are in real life conditions, you would think.

However, this is what they don’t do.  During the testing process in most parts of the world, the testing gets done in a lab under controlled conditions.  It’s like the experiments we did in science class at school, where there’s only one variable to be tested and everything else is exactly the same. This does mean that the fuel economy stats aren’t going to be skewed by things like a headwind during the testing process so you can compare car with car, but it’s still a bit disappointing for the average driver.

During the test in the lab, the vehicle gets put on a dynamometer or a rolling road for about 20 minutes.  The temperature is somewhere between 20°C and 30°C, and the cars being tested have been nicely run in and are tested from a cold start.  During the test for urban figures, the car “drives” for 4 km with a maximum speed of 50 km/h, a few stops and a fair bit of idling, for an average speed of 19 km/h.  For the open road speed, the car covers 7 km, gets up to a maximum of 120 km/h and averages 63 km/h.  Each test gets repeated a few times – about four times, according to one source.  To get the combined figure, they get the average of the two figures weighted by the distance covered in each test.   OK, this is a fairly simplified description of the procedure, and if you want to know all the details and all the maths, you can read it at

The regulations state that “Only the equipment necessary for the operation of the vehicle during the test shall be in use.” They also state that the air con needs to be switched off and the heating should be at “normal” (like you do when driving around when the thermometer hits 27°C?).  The widest tyres should be used.  The lights and indicators should be off.  The slope of the road should be no more than 2%, the top wind speed should be 3 m/s on average (that’s about 11 km/h) and the track should be dry.  Tyre pressure should be whatever’s specified by the manufacturers (and you can bet that they’ll put it on the harder side during testing to get more frugal figures). Need we add that there’s probably only going to be one person in the car (unless they get a robot to do it somehow) and the boot will be empty?

Yes, well, we all drive on perfectly dry roads that are practically flat on windless days with nothing in the boot and with the air con off and the windows up (on a hot day?) and the mirrors folded back as sleekly as possible to minimise drag.  And we don’t touch the indicators or the lights at all.  Which is what the manufacturers were doing when they got the test stats so they could get them looking as attractive as possible.

Back in the real world, you’re going to have wet roads, slopes and headwinds.  You’re going to use the headlights and the indicators.  You’re going to have kids, the dog and the groceries loading up the car adding to the weight and thus the amount of energy needed to move the car.  You’re going to have the air con on (or you’re going to open the window at lower speeds) to keep cool.  You’ll put on the radio (and have the aerial up – you can bet that they’ll put it down during the tests to reduce even the teeniest bit of drag).  You’ll plant the right foot occasionally to nip into a gap or to overtake.  You’re going to idle more than the urban test does, and you’re going to average higher speeds on the open road than what happens in the test. So you’re not ever going to get the same fuel economy figures that the brochure or the car review tells you.

The moral?  There are two of them.  Number 1 is to read the fuel economy figures by all means – they’ll help you compare car with car (although common sense tells you that a little Honda Jazz  is going to have better fuel economy than a monstrous big HSV ).  But take them with a grain of salt.  Number 2 is to make your driving as close as possible to test conditions… But use the indicators and the headlights – please!

Safe and happy driving,