How to calculate fuel consumption

Q: How to work out fuel consumption?

A: When you look at the official government website, fuel consumption is usually expressed as a number of litres of fuel per 100km travelled. But it is also better understood by some (often older folk who remember miles per gallon) when it’s expressed as a number of kilometres covered per litre of fuel. Either way, it’s pretty simple mathematics.

In an age when even budget new cars have trip computers to offer up an average fuel consumption number on the run, it’s tempting to assume that that’s all you need to check.

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Which is fine if you’re not too concerned about pin-point accuracy, because on-board trip computers aren’t as accurate as many people think.

These computers typically measure the amount of air that enters the engine (via an engine sensor) and then refer to a set of tables that gives it the corresponding amount of fuel.

Unfortunately, this isn’t always a totally accurate method and while it’s convenient and close enough for most, it’s not the way a scientist would measure fuel consumption.

That same scientist would resort to basic mathematics, and regardless of whether it’s to calculate how many litres petrol per kilometre, how many litres per 100km or even kilometres per litre, it’s all the same fundamental but specific fuel consumption formula.

So, here’s how it works: We have two known factors here. The first is the number of kilometres travelled (assuming you reset your trip meter when you last fuelled up) and the number of litres you’ve just added to the tank to bring the car back to full.

On-board trip computers aren’t as accurate as many people think.

To keep the mathematics simple, let’s say you’ve travelled 500km from the last fill and added 40 litres to bring the tank back to full.

If you want to know the litres per 100km, you divide the number of litres by the number of 100km chunks of travel (in this case, five - 500 divided by 100). When you divide 40 by five, you get eight which means you’ve used eight litres per 100km travelled.

You’ll probably find your chunks of 100km aren’t a nice, round number, but it works with decimal places, too. So, if you covered 560km for those same 40 litres, the equation would be 40 divided by 5.6 (for a figure of 7.14 litres per 100km).

But what if you want to know the kilometres per litre you’ve achieved on the same trip? Now the equation becomes the distance travelled divided by the litres consumed.

That is: 500 divided by 40, which means you’ve covered 12.5 kilometres for every litre used.

The biggest factor determining fuel economy is your driving style.

While this is often a more accurate method that relying on the trip computer, there are still variables.

Odometers are not always spot-on accurate, and while fuel pumps are regularly checked for accuracy, some will still fill a car more completely than others before 'clicking off'.

Perhaps the forecourt where you’re filling isn’t perfectly flat. Heck, even ambient temperature can have an effect on how much physical space petrol occupies.

The next step, of course, is how to work out fuel cost as opposed to the bald numbers. Again, this is pretty simple and based on our previous figures and a per-litre fuel price of $1.80, our car that achieves 8.0 litres per 100km costs $14.40 to cover that distance. Per kilometre, that works out to 14.4 cents.

In any case, the biggest factor determining fuel economy is your driving style. It doesn’t matter whether you drive a car, ute, SUV or truck, or whether it’s powered by petrol, diesel, gas (LPG) or it’s a hybrid, the best way to save fuel is to drive smoothly and use as little throttle as possible.

Even ambient temperature can have an effect on how much physical space petrol occupies. (Image: Matt Campbell)

The other number that can be a bit misleading in fuel consumption terms is the official government-test figure that is on the windscreen of brand-new cars in the showroom. These figures equate to a highway cycle test, an urban cycle test and a combination of those two tests.

The tests are exactly the same for each and every vehicle sold in Australia, but while they can therefore be compared against each other, in terms of real-world fuel consumption results, they are a long way off.

Broadly speaking, in normal use, you’ll never match those numbers, let alone beat them. Why? Because car makes understand the testing procedure intimately and actually design their cars to do well in the test, often at the expense of real-world driveability and fuel economy thanks to super-tall gearing and other measures.

You can also thank these tests for the often-annoying stop-start function on new cars which saves precious drops on the test, but is something many owners switch off every time they drive the car.

That’s why the car that suggests (on the test sticker) that it can cover 100km on a highway on just eight litres of fuel, will probably require more like 10 litres to actually do so outside the test lab.

Use these figures only as an apples versus apples measurement yardstick against competing makes and models where they’re relevant, and not as a real-world guide to consumption.