Why is fuel economy so hard to compare?

Your name is Fred, and you own an older 2.0-litre car that’s supposed to use as little fuel as 8.0L/100km, but the on-board trip computer never posts a figure lower than 11.0L/100km.

Why is that?

There are three basic reasons for fuel economy in the real world varying dramatically from results of one or other of the lab tests auto manufacturers rely on when publishing official figures. It could be due to:

In the case of Fred’s car, the underlying problem is environmental. The car starts from cold and travels to work 15 minutes away, where it’s then parked for several hours. Long after the engine has cooled again, it’s restarted and driven home for the evening.

In each short trip, the car only reaches optimal operating temperature perhaps five minutes into the journey. In other words, the engine is running richer and the lubricant is sludgier (the transmission fluid and diff oil as well) for around 30 per cent of the journey’s duration. Tyre pressures might be lower too, until the air inside heats up and expands.

Why does the manufacturer claim that the car is much more economical than Fred’s ‘real-world’ experience indicates? Quite simply it’s because Fred’s car can actually be that economical in other circumstances.

On the open road, over a longer journey – and with the engine already warm before the start of the trip – Fred’s car can post fuel consumption as low as 6.5L/100km, which is definitely better than the applicable ADR figure.

The current Australian Design Rule that covers fuel economy and emissions data labelling for vehicles under 3.5 tonnes gross vehicle mass is ADR 81/02.

Official figures quoted for new cars are usually ‘combined-cycle’ numbers; they’re sort of an average of ‘extra-urban’ (emulating open roads, such as freeways) and ‘urban’ (a laboratory analogue for lots of traffic lights, give-way signs and roundabouts). These numbers rarely reflect the personal experience of drivers in their own cars.

A car spending a lot of time on freeways will post fuel economy figures typically lower than the ADR combined-cycle number for the car. The same may be true of cars that only hit the road at weekends for a gentle run in the country.

A more appropriate ‘official’ figure for these cars is the ‘extra-urban’ number, rather than the combined-cycle figure.

That country drive can mean different things to different people, of course. A visit to a wine-growing region at the end of a long, straight rural highway is not the same as a merciless flogging along a coastal road with plenty of twists, turns and uphill climbs.

Back in town, someone commuting in heavily congested traffic every day will use much more fuel than the combined-cycle testing figure. For these commuters, the ADR-approved ‘urban’ cycle is the more appropriate guide to compare vehicles.

And that is the whole purpose of official fuel economy figures; they’re supposed to provide consumers a means of comparing apples with apples when those consumers are planning to buy the most fuel-efficient vehicle for the money.

If you know that you spend more time in traffic and you’re cross-shopping two small cars, take a look at the federal government’s Green Vehicle Guide, and compare the vehicles on your shopping list using the urban-cycle figures, not the combined-cycle data.

Conversely, if you spend more time on the open road, use the GVG’s ‘extra-urban’ figures to align with what you expect to see in a small car, a medium SUV or a dual-cab 4x4 ute – whichever type of vehicle you plan to buy.

Environmental factors aren’t limited to the road network alone. A cold climate can keep a car from reaching normal operating temperature, and a drive up Mount Wellington in the middle of winter is bound to use more fuel, especially if you have the climate control working hard to keep the passengers warm.

Equally true, a hot day in the tropics will have the climate control working overtime, and that will have some impact on your car’s fuel economy in the longer term as well.

If you insist on using launch control at every set of traffic lights, you would be insane to expect fuel economy anywhere near the manufacturer’s official combined-cycle fuel consumption figure. Even the urban figure is likely out of reach.

But using more fuel isn’t necessarily due to just extreme driving. Any sort of bad driving habit can result in heavier fuel use.

Perhaps you leave your manual car in third gear at the 60km/h speed limit. Simply shifting up one gear could make a significant difference – and reduce your fuel consumption to much nearer the official number.

It can work the other way too, if you slip a car into a higher gear at 1500rpm, the engine will labour and may use more fuel than leaving it in the lower gear.

Do you keep your foot hovering over the brake pedal just in case someone cuts in front of you or a pedestrian steps out? If the brake pads are making contact with the rotors a lot of the time, that will use extra fuel too.

Do you start the engine and warm it up for minutes on end in the morning?

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Do you brake late and hard, when coasting up to the lights might save you from coming to a complete halt?

There’s a fairly exacting science to driving economically, and many people just don’t realise that they’re not applying that science correctly in their day-to-day motoring. The difference between driving for economy and the alternative can be substantial over a longer period of vehicle ownership.

And in case you’re wondering, here are several reasons why motoring journalists record bad fuel economy:

The third factor that will influence the fuel consumption of your vehicle is the car itself. If tyre pressures are low, if the spark plugs have never been changed, if the air filter contains more dust than a pharaoh’s tomb, you will never see anything like combined-cycle fuel economy from this car.

Nor will you save fuel by transporting the whole family and luggage every time the car leaves the garage. If it’s towing a caravan or ski boat as well, the fuel consumption will be even higher.

And higher fuel consumption could also be due to the very tight engine tolerances in a new car. Family cars will often become slightly more economical over time, as the mechanicals loosen up.

Regular maintenance will make a difference, as will removing any items that you don’t need in the car. But for workhorses that tow, or family cars that do transport more than one person, heavier fuel consumption is just a fact of life, unfortunately.

So to recap, here’s why official fuel economy figures are ‘ridiculous’:

Motorists have often expressed their mistrust of official fuel economy figures in the past, but that mistrust received a kick along when lobby groups in Europe revealed that the car companies were using every available loophole in the testing process to exaggerate the car’s fuel economy.

In some cases, the car companies were going way beyond just exploiting loopholes in the test procedures; they were specifically setting up cars with software to run very lean in a test situation – the engine-management algorithm distinguishing between testing and on-road use, and readjusting the engine tune accordingly, on the fly.

As a consequence, the European Union replaced its previous NEDC fuel economy and emissions testing program with WLTP (Worldwide harmonized Light vehicles Test Procedure), a program aimed at replicating real-world fuel consumption and emissions in the laboratory. It’s a significant improvement on the NEDC procedure.

In Australia, ADR 81/02, which previously adopted NEDC figures for European imports, moved to WLTP figures with effect from May 2019.

It means that new models of cars released here since then will be rated for fuel economy and emissions based on WLTP results, not the old NEDC results.

But for some cars that were sold here prior to May 2019, and are still on sale, there’s a very strong likelihood that the ADR 81/02 fuel economy and emissions figures will be based on NEDC, not WLTP.

To illustrate what that means, note the case study for the Range Rover Sport in this advice feature from 2018.

If there’s that much variance in the laboratory, imagine how much more potential for variance there is out on the road!