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Fuel efficiency ratings | what do they tell you?

Fuel consumption label that Federal law requires be stuck to the windscreen of new cars.

Just what do those numbers on the windscreen mean and where do they come from?

It sounds like one of those desperately dull jobs that you're glad someone else is out there doing.  Surely, to come up with those official fuel-consumption numbers we so often hear quoted with new cars - or read on the label that Federal law requires be stuck to the windscreen of new cars - there must be a fleet of people driving around very slowly and carefully.

How else do car companies arrive at these numbers, telling us how many litres of petrol or diesel we'll use in the various modes - urban, extra urban (that's the highway) and combined?

You might be surprised to hear that those figures are actually obtained by car companies putting their own vehicles on a dynamometer for 20 minutes and "simulating" a drive through the "urban" city (average speed 19km/h), on an "extra urban" freeway (top speed a daring 120km/h), with the "combined" figure calculated by simply averaging the two results. This might put an end to any mystery around why you can't actually achieve the claimed fuel figure in real life.

They do try to make the test - which is dictated by Australian Design Rules and based on procedures used by the United Nations Economic Commission for Europe (UN ECE) - as realistic as possible by simulating aero drag and inertia and using a fan to imitate airflow over the car's front end. one can spend more money to get a better result, and thus it allows you to compare apples with apples

As BMW spokeswoman Lenore Fletcher explained it to us, because everyone has to do the same test, and it's so strictly controlled, no one can spend more money to get a better result, and thus "it allows you to compare apples with apples." 

Even if those apples might not be quite as juicy when you get them home. "The figures are only indicative, of course, but you'll find that BMWs get a lot closer to the claimed figures than many other companies, thanks to our engine technology," Ms Fletcher said. "By driving carefully and using the EcoPro modes, we have actually achieved those quoted numbers in the real world on many of our cars."

Ashley Sanders, manager of certification and regulation compliance for Mitsubishi Australia, says obtaining the fuel figures is a time-consuming and expensive exercise, and that people need to remember that the numbers they achieve in their own cars are heavily influenced by where, and how, you drive.

The ADR cycle is a comparative tool rather than a definitive one

"If you accelerate hard and use more energy, you will get a higher figure," Mr Sanders said. "You can't hope to get what's on the ADR when you're towing a caravan and driving up hills. If you explain it to people like that, why would you expect to get the same sort of fuel consumption value if you're driving in a way that's different?

"The ADR cycle is a comparative tool rather than a definitive one." Mr Sanders is now dealing with a whole new level of complexity, because his company offers plug-in hybrid electric vehicles (or PHEVs), like the Mitsubishi Outlander PHEV, which claims a combined fuel figure of just 1.9L/100km. 

So how do they come up with that? "The test for PHEVs is the same, done to the same drive cycle, but you do it twice, first with a car that's fully charged to 100 per cent, and you do the European drive cycle and you measure the fuel consumption and the energy consumption," Mr Sanders explained.

"Then you repeat with the battery in a zero per cent state of charge, and you weight the two figures through a calculation provided in the ADR and come up with a weighted value."

PHEVs run in various different ways, but it's not uncommon for them to survive the first drive cycle test on electric power alone, thus registering a zero fuel usage figure. The second test forces the hybrid systems into action.

"The difficulty with PHEVs at present is there's not many to compare against, so people attempt to compare the figures from these hybrids with a petrol or diesel car, and that really is comparing apples with oranges," Mr Sanders said.

"The 1.9L/100km is fantastic figure, especially for an SUV, but if you're someone who only drives to work and back each day, and that trip is less than 40km, you might only need to charge it up every couple of days, and your petrol use for the week would be zero."