Fuel efficiency explained

1 July 2014

We’ve all heard the term fuel-efficient bandied about and although it’s a very common term when talking about cars, what exactly does it mean?

At the very basic level, fuel-efficiency refers to the distance you can travel on a amount of fuel. Let’s break it down.

  Car A Car B
Yearly driving average   14,750km   14,750km
Fuel economy   4-litres/100km  5-litres/100km
Litres required   590 litres  737 litres
Fuel cost for year (@ $1.25/litre) $737.50   $921.25


Even with a 1-litre difference in fuel efficiency, the owner of Car B will need another 147 litres of fuel and spend an extra $183.75 than the owner of Car A. All that from an extra 1-litre per 100km.

What qualifies as fuel efficient, then?

Anything that is listed as less than 6-litres/100km or more than 16.5km/1-litre is considered to be pretty good.

Fuel efficiency measurements 

The first (and most common) reference is litres per 100km (litres/100km). This is how many litres of fuel the car needs in order to travel 100km. You’ll often see it referred to as ‘fuel economy’. Rule of thumb on this one: the lower the number of litres stated, the better the fuel economy.

Not as frequently used in Australia, kilometres per litre (km/1-litre) is another way to look at a cars fuel consumption. In this instance, the higher the number of kms travelled the more fuel-efficient.

Petrol vs diesel?

Petrol is the most widely-used fuel in Australia, however diesel is becoming more popular. In the last 10 years, diesel sales have almost doubled and the idea that diesel is reserved for dirty smelly trucks has long gone.

Generally speaking, diesel engines are more fuel efficient than petrol engines. They use compression ignition (instead of spark plugs) which results in a more efficient fuel-burn. This requires a more high-tech engine design, which is one of the reasons why diesel cars cost more than petrol.

Modern diesel engines are often turbocharged and have a lot of pulling power (AKA torque). It can shift through gears more quickly than a petrol engine and runs at a lower RPM. All of this means it uses less fuel at each stage than an equivalent petrol engine would.

What about LPG?

LPG presents many benefits, but strictly speaking, fuel economy is not one of them. In terms of cost, you’re getting a more economical deal with LPG but fuel consumption after an LPG conversion can increase by up to 40%, so you’ll be filling up more often. However, LPG can be up to 60% cheaper than petrol, litre for litre. So the consumption increase is more than covered by the cost savings.

What kind of fuel do you use? Tell us your experience in the comments below.

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