• Find your car
• Sell or trade in
• Car finance
• Why Carma?

When buying a used car, a complete service history is one of the clearest indicators of whether these systems have been kept on top of. A well-documented car is less likely to have accumulated the kind of deferred maintenance that quietly drives up running costs.

You're not using the right fuel grade for your car

Every car has a minimum recommended fuel grade printed on the fuel cap and in the owner's manual. Using a lower grade than your car needs costs you efficiency in two ways.

If your car is designed for 95 or 98 RON and you're filling with 91, the engine management system retards ignition timing to prevent knock, which costs both performance and fuel economy. If you're using E10 in a car that recommends 91 or higher, you're compounding the problem: E10 has lower octane than 91 RON and lower energy density, meaning your engine burns more of it to travel the same distance. The RACQ estimates E10 results in approximately 2–3% higher fuel consumption than 91 RON over the same distance. In a car specced for 95 RON, E10 is a double disadvantage.

The cheapest fuel at the bowser is not always the cheapest fuel per kilometre. Check what your car actually needs and run accordingly.

Roof racks and extra weight add up fast

Roof racks, bike carriers, and cargo pods dramatically increase aerodynamic drag, particularly at highway speeds. Independent highway testing has found that empty roof racks can increase fuel consumption by up to 15% at freeway speeds. If yours aren't in regular use, removing them between trips is one of the easiest efficiency gains available.

Excess weight in the boot tells the same story. Every unnecessary 45kg adds measurable load on the engine over time. Boots used as permanent storage are more expensive than they appear.

Two causes you probably never knew about

A faulty oxygen sensor is one of the most commonly overlooked causes of excess fuel consumption. The O2 sensor tells the engine management system how much oxygen is in the exhaust, which determines how much fuel to inject. A sensor that degrades gradually can cause the system to over-fuel without crossing the threshold that triggers a check engine light. The car may drive normally for months while burning more petrol than it should.

The other: engine oil grade. If your car specifies 5W-30 and it has been filled with 10W-40, the thicker oil creates additional internal friction the engine works against on every drive. The effect is modest on its own, around 1–3%, but it stacks with everything else on this list. The correct grade is on the filler cap of most modern cars and costs nothing extra to insist on at a service.

If you've fixed all of this and the numbers still don't add up

Some petrol consumption issues aren't about habits or maintenance. They're about the car itself. Older engines, larger-displacement petrol SUVs, and vehicles with higher by-design running costs simply consume more fuel, regardless of how carefully they're driven.

Used hybrid cars have become the most practical way to cut fuel costs without changing how you drive. A used Toyota RAV4 in hybrid form is officially rated at around 4.7–4.8L/100km under test conditions, with real-world mixed driving typically sitting in the 5.5–6.5L/100km range — still roughly half the consumption of a comparable petrol SUV. For buyers who want something smaller, the used Toyota Yaris Cross and used Mazda 3 are among the most fuel-efficient options in the Australian used market.

If the weekly fuel bill has become the deciding factor, it might be worth seeing what a more efficient car would actually cost you across a full year.