Fuel efficiency ratings | what do they tell you?

Just what do those fuel consumption ratings numbers on the windscreen of new cars 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 avg fuel-consumption numbers we so often hear quoted with new cars - or read on the ADR 81/02 fuel consumption 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 official fuel consumption figures, telling us the car CO2 emissions and how many litres of petrol or diesel we'll use in the various modes - urban, extra urban (“extra urban” fuel consumption relates to use on a highway) and combined (which finds the average of the “city vs highway” urban and extra urban numbers)?

You might be surprised to hear that those figures are actually obtained by car companies putting their own vehicles on a dynamometer (a kind of rolling road, like a treadmill for cars) 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 fuel economy "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, aiming to ultimately offer up accurate fuel efficiency ratings on the fuel consumption label in Australia.

As one industry expertexplained 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. Here is how typical spokesman, from BMW Australia, addresses that question of official figures not matching those achieved in the real world: “The combination of highly efficient engines and intelligent electronic powertrain management enables us to be in full accordance with regulatory requirements while also achieving the best possible results for our customers.”

Truly, a politician could not have said less any better.

Fortunately, James Tol - Manager, Certification and Regulatory Affairs for Mitsubishi Australia - was significantly more forthcoming. Mitsubishi, of course, has even more complexity to deal with, because it offers plug-in hybrid electric vehicles (or PHEVs), like the Mitsubishi Outlander PHEV, which claims a combined fuel economy figure of just 1.9 litres per 100km. 

“Obtaining the fuel figures is a time-consuming and expensive exercise, and people need to remember that the numbers they achieve in their own cars are heavily influenced by where, and how, they drive,” Mr Told explained. 

“They will also be influenced by what accessories you may have fitted to your vehicle, how much weight you are carrying, or if you are towing.

“There has been a lot of debate about the merits of laboratory-based fuel-consumption testing, and how it relates to real-world driving. Improvements have been made to laboratory testing in Europe that seek to more closely represent real-world conditions. These new procedures have not yet been adopted into Australian regulation. 

“However, by necessity, it remains a laboratory test, and individuals may or may not achieve the same results when driving in the real world.”

As he points out, laboratory tests ensure repeatable results and ensure a level playing field on which to compare different makes and models. They are comparative tools, not definitive tools.

“PHEV’s are sometimes reported as having significant deviations in ‘real world’ use. I guess in this regard PHEV’s are an easy target for a headline under the current test. It comes back to the declared figure being a comparative tool based on a prescribed driving route, with a defined length and set of variations, rather than a definitive result based on real-world experience,” Mr Tol adds. 

“Over a weekly commute, with regular charging, depending on commute distance and your driving style, it is entirely possible to use no fuel at all. 

"Over a longer trip, or if the battery has not been recharged, a PHEV’s fuel use will be more like that of a normal (non-plug in) hybrid. This range of performance is not captured within the single declared figure that must be stated in accordance with regulation. 

“As a comparison tool, however, the declared figure certainly can give a guide as to comparative performance with other PHEVs.”