Is Your Car Wasting Fuel? Here's What to Check

Somewhere between the rising bowser price and your monthly bank statement is an uncomfortable realisation: you're spending more than you think you should on running your car, and you're not entirely sure why.

The thing is, most cars that are running less efficiently than they should aren't doing anything dramatically wrong. There's no smoke, no warning light, no obvious mechanical problem. They're just quietly bleeding money through a combination of maintenance issues, driving habits, and small overlooked details that individually seem minor but add up to a meaningful annual cost.

This guide works through every area where your vehicle might be wasting money , from the easiest checks you can do yourself in five minutes to the mechanical issues worth getting a professional opinion on. Think of it as an audit of your car's efficiency, designed for Australian conditions and Australian drivers.

First: How to Know If Your Car Is Actually Wasting

Before you can fix a problem, you need to know it exists. The most reliable way to check your real-world consumption is simple:

•       Fill the tank completely to the first click of the nozzle

•       Reset your trip odometer to zero

•       Drive normally , don't change your habits , until the next fill-up

•       Fill completely again and note exactly how many litres went in

•       Divide litres used by kilometres travelled, then multiply by 100

•       This gives you your actual L/100km

Compare this figure to your manufacturer's stated combined cycle consumption (in your owner's manual or on the manufacturer's website). Real-world consumption is typically 10–20% above the manufacturer's test figure , that's normal. If you're sitting 25–35% above the stated figure, something is likely contributing to excess consumption.

Example: if your Mazda CX-5 2.5L states a combined consumption of 8.0L/100km and you're regularly seeing 11–12L/100km in mixed driving, the gap beyond normal variation suggests there are specific factors worth investigating.

Check 1: Your Tyres , Start Here Every Time

Tyre issues are the most common cause of hidden fuel waste in Australian passenger vehicles , and they're free to fix. The fact that most people don't check their tyre pressure monthly makes this a near-universal opportunity for improvement.

Tyre Pressure: The Free Fix

Under-inflated tyres create greater rolling resistance. The tyre flexes more than it should with each rotation, generating heat and absorbing energy that should be propelling the car forward. The relationship between under-inflation and consumption is well established:

Pressure Deficit	Extra Rolling Resistance	Consumption Penalty
5% below spec (e.g. 29 vs 30.5 PSI)	~1.5%	~1%
10% below spec	~3%	2–3%
20% below spec (common on neglected vehicles)	~6%	5–7%

How to check correctly: always check when cold , before driving or less than 3 km from home. Warm tyres read higher than their actual cold pressure, giving you a false sense of security. The correct pressure is on the tyre placard inside your driver's door jamb , not on the tyre sidewall, which shows maximum pressure.

•       Check all four tyres monthly

•       Use a digital tyre gauge , available for $12–$15 from any auto parts store, far more accurate than servo gauges

•       Inflate at a servo (most are free to use in Australia) or with a portable compressor

Wheel Alignment: Often Overlooked

If your wheels aren't pointing in precisely the right direction, the tyres scrub against the road surface instead of rolling cleanly through it. This creates continuous rolling resistance that costs both fuel economy and tyre life.

Signs your alignment needs attention: the car drifts to one side without steering input, the steering wheel isn't centred when driving straight, or you see uneven tyre wear , more worn on the inside or outside edge than the centre tread.

•       A wheel alignment costs $60–$90 at most tyre shops

•       Misalignment severe enough to cause consumption penalties of 5–8% is not uncommon after hitting kerbs, deep potholes, or after suspension component wear

•       Should be checked after any significant impact and at least every 20,000 km as routine maintenance

Check 2: Under the Bonnet , What to Look At

The Air Filter

Pop the bonnet and locate the air filter housing , typically a black plastic box connected to the intake system by a large hose. Undo the clips, pull out the filter element, and hold it up to the light.

A new filter is white or light grey and you can see light through it clearly. A used but acceptable filter is darker but still reasonably open. A filter due for replacement is dark brown to black, visibly clogged with debris, and you can barely see light through it.

Why it matters: a clogged air filter starves the engine of the air it needs for combustion. The fuel management system compensates, the engine struggles, and consumption goes up. A significantly dirty filter can add 6–11% to fuel consumption.

•       Replacement cost: $20–$45 for most Australian passenger vehicles

•       Service interval: typically 20,000–40,000 km, but inspect at every service regardless , dusty driving conditions can clog a filter in half that time

•       For any Aussie driver who regularly travels on unsealed roads, the interval should be halved

Engine Oil Level and Condition

Pull the dipstick, wipe it clean, reinsert fully, and pull again. The oil level should sit between MIN and MAX. Then look at the oil itself:

•       Healthy oil: amber to light brown, translucent when held up to light

•       Aging but acceptable: darker brown, less translucent

•       Overdue for change: black, opaque, possibly with a burnt smell

•       Concerning: milky or cream-coloured (suggests coolant contamination , get this looked at immediately)

Degraded oil has higher viscosity and creates more internal friction. Fresh oil of the correct grade flows freely, coats bearing surfaces optimally, and lets the engine operate with minimum internal resistance. The consumption improvement from an oil change in an overdue engine is typically 1–3%.

Also check that the oil level isn't dropping faster than expected between services , this indicates either an external leak or internal oil consumption, both worth investigating.

Coolant Level and Thermostat Function

Check the coolant expansion tank when cold , the level should be between MIN and MAX. Low coolant can affect engine operating temperature.

More importantly, watch your temperature gauge for the first 10 minutes after a cold start. It should rise steadily to its normal operating position and stay there. If it never quite reaches normal position, or climbs very slowly, the thermostat may be stuck in the open position , allowing coolant to flow continuously and preventing the engine from reaching optimal temperature. A cold-running engine uses more fuel on every drive.

Check 3: What Your Dashboard Is Telling You

Modern vehicles are remarkably good at communicating problems. Ignoring dashboard indicators is both expensive and avoidable.

The Check Engine Light

The engine management light (check engine, engine symbol) is one of the most commonly ignored indicators on Australian roads. Many drivers know it's on but assume nothing dramatic is happening because the car 'drives fine'. The problem is that the faults most likely to trigger this light without causing obvious drive symptoms are exactly the ones that quietly hurt fuel economy:

•       Oxygen (lambda) sensor fault , causes the engine to run with an incorrect air-fuel mixture, often rich (too much fuel per combustion event). Economy can drop 10–20%

•       Mass airflow sensor fault , the sensor measuring incoming air is providing inaccurate data, causing incorrect fuelling

•       Evaporative emissions system (EVAP) fault , prevents the fuel tank vapour system from functioning correctly; can cause raw fuel vapour to be vented rather than recirculated

•       Misfire codes , a cylinder that isn't firing reliably is wasting the fuel injected into it

Getting a check engine light diagnosed is inexpensive , $50–$100 at most independent workshops for a scan. Many auto parts stores in Australia (Repco, Supercheap Auto) will read the code for free. The code doesn't tell you exactly what's wrong, but it tells you exactly where to look.

Fuel Economy Readout

If your vehicle has a real-time fuel economy display , most cars sold in Australia in the past decade do , spend a few days paying attention to it. The instantaneous L/100km display won't give you a precise overall picture, but the average consumption display (typically reset each trip or each tank) is a useful baseline.

If the displayed average is significantly above your previous norm, the car is flagging the problem for you. Note when it changed , that timing often points toward the cause.

Check 4: An Honest Look at How You're Driving

This one requires a bit of self-reflection, but it's worth it , because driving style can account for a 20–30% variation in consumption on the same vehicle on the same route.

Acceleration Style

Every time you accelerate aggressively from a stop or low speed, the engine is demanding maximum fuel delivery. This is the single most fuel-intensive thing a normal passenger car does. Smooth, progressive acceleration from lights and junctions uses a fraction of the fuel that aggressive acceleration does , and in urban traffic, you arrive at the next red light at essentially the same time.

How You Approach Traffic

Braking late and hard converts momentum , which you paid for with fuel , into heat through your brake pads. That energy is gone. Lifting off the throttle early when you see a stop coming and coasting to a gentle deceleration recovers that momentum as usable forward motion. Modern fuel-injected cars use almost zero fuel on overrun, making coasting one of the most efficient things you can do.

Highway Cruise Speed

Aerodynamic drag increases with the square of speed. Driving at 120 km/h creates 44% more aerodynamic drag than driving at 100 km/h. Driving at 130 km/h nearly doubles it. The fuel required to overcome that drag scales accordingly.

If you regularly cruise at 120–130 km/h on highway runs, reducing to 100–110 km/h will produce a measurable economy improvement , typically 10–20% on that portion of driving. On a regular Sydney to Canberra or Melbourne to Geelong run, that's a meaningful amount of money saved over a year.

Air Conditioning Habits

Running the AC at maximum cooling in city traffic adds 10–15% to consumption. A few adjustments reduce the penalty:

•       Use recirculation mode once the cabin has cooled , the AC compressor works far less hard cooling pre-cooled air than drawing in hot outside air

•       Pre-cool by opening windows briefly before running AC from cold

•       Below 60 km/h in moderate temperatures, windows are often more efficient than AC; above 80 km/h, AC is usually more efficient than the drag from open windows

Check 5: What You're Carrying and What's Attached

This one gets ignored because it feels too obvious , but it's genuinely costing Australian drivers money every day.

Boot Clutter

Every 50 kg of unnecessary weight in the car increases fuel consumption by approximately 1–2% in urban driving. Open your boot right now. Tools you haven't touched in months. Sports gear that 'might' be needed. Shopping bags, old service items, emergency equipment that rarely gets used.

Do a declutter. If you haven't used something in the past month, it shouldn't be in the car. For a boot carrying 80 kg of miscellaneous gear, clearing it out saves $40–$80 per year in fuel alone.

Roof Racks and Accessories

An empty roof rack increases aerodynamic drag by 5–10% at highway speeds , you're pushing a large metal frame through the air for no payload benefit. A cargo box or pods on a roof rack, even empty, can increase consumption by 20–25% at highway speeds.

If you use the roof rack twice a year for camping and drive on the highway five days a week, the maths heavily favour removing it between uses. Most roof racks take less than five minutes to remove.

•       Bull bars add both weight and frontal area , if you don't need one for current driving conditions, consider removing it

•       Towbars create aerodynamic disruption even when not towing , on purely highway driving, a removable towbar is worth removing for long trips

Check 6: How and When You're Buying

Using less of it per kilometre is the main game, but paying less per litre for what you do use has the same net effect on your wallet. The gap between buying well and buying poorly at Australian bowsers is larger than most people realise.

The Weekly Price Cycle

Petrol prices in Australian capital cities , Sydney, Melbourne, Brisbane, Adelaide, Perth , follow a structured weekly cycle. Prices drop sharply to their lowest point on a consistent day (typically Tuesday or Wednesday in most cities) and rise gradually to a peak by the weekend.

The swing from bottom to peak can be 20–30 cents per litre in volatile weeks. On a 60-litre fill-up, the difference between buying on the cheapest versus most expensive day is $12–$18. Across 20 fill-ups per year, that's $240–$360 saved purely by timing , no driving changes required.

Price Comparison Apps

The price difference between servos within the same suburb on the same day can be 15–20 cents per litre. There is no reason to pay peak price when the cheapest option is 2 km away.

•       MotorMouth , shows real-time prices and weekly cycle patterns by city; the most widely used option in Australia

•       GasBuddy , crowd-sourced price data; good for real-time spot comparisons

•       FuelWatch (WA only) , government scheme showing tomorrow's prices 24 hours in advance

•       Google Maps and Apple Maps , both display live petrol prices at nearby stations in major Australian cities

Using the Right Grade

If your vehicle specifies 91 RON, using 95 or 98 provides no benefit , you're paying a premium for octane resistance your engine cannot use. Over a year of driving, this unnecessary upgrade can cost $100–$200 with zero return.

Conversely, using 91 in a vehicle that requires 95 causes the engine's knock sensor to retard ignition timing , reducing efficiency and potentially offsetting the price saving with worse economy. Check your fuel filler flap , the correct grade is almost always printed right there.

Your Complete Fuel Waste Checklist

Work through this systematically. Tick off what's fine, investigate what isn't:

Check Item	What to Look For	Estimated Annual Saving if Fixed
Tyre pressure (all 4)	At correct cold pressure per placard	$55–$150
Wheel alignment	No pulling; centred steering; even tyre wear	$100–$200+
Air filter condition	White/light grey; light passes through	$150–$300 (if dirty)
Engine oil level and colour	Between MIN-MAX; amber-brown colour	$60–$130 (if overdue)
Check engine light	No lights showing	$150–$500+ (depends on fault)
Temperature gauge behaviour	Reaches normal position within 10 min of start	$100–$200 (if thermostat faulty)
Acceleration style	Smooth and progressive, not aggressive	$270–$450
Highway cruise speed	100–110 km/h rather than 120–130	$200–$500 (highway drivers)
Boot clutter	Empty of non-essentials	$40–$80
Roof rack / accessories	Removed when not in use	$80–$200 (highway use)
Buying on cheapest day	Using price app; filling on low day of cycle	$240–$360
Using correct octane grade	Grade on fuel filler flap is what's in tank	$100–$200 (if using wrong grade)

When to Stop Checking Yourself and Call a Mechanic

Most items on the checklist above are things you can assess and address yourself, or at a tyre shop. But some efficiency issues are caused by mechanical faults that require professional diagnosis and repair. Get a mechanic involved when:

•       There's a check engine or engine management light showing , get it scanned; the fault code points directly to the cause

•       The car is running rough, misfiring, hesitating under acceleration, or is hard to start , these are combustion and ignition issues

•       You can smell burning from around the wheels after driving , this is a dragging brake calliper, a safety issue as well as an efficiency one

•       The temperature gauge is behaving abnormally , either running cold consistently or climbing higher than normal

•       You've worked through the checklist and consumption is still noticeably worse than your established baseline , there's likely a fault that isn't visible without diagnostic equipment

When you visit a mechanic, tell them your current L/100km, your previous normal consumption, when the change became noticeable, and any other symptoms. This information makes diagnosis targeted rather than speculative , saving both time and money.

An Australian Note: Conditions That Make This More Important

Australia's driving environment accelerates some of these checks and makes others more urgent than they would be in other markets:

•       Summer heat , sustained high ambient temperatures accelerate oil degradation, increase tyre pressure variation (hot air expands inside the tyre; cooler overnight temperatures drop it below recommended), and work the cooling system harder. Check both during extended heat periods

•       Dust , outback, rural, and construction-area driving loads air filters extremely rapidly. If you drive on unsealed roads regularly, inspect the filter every 5,000–8,000 km regardless of the service schedule

•       Long regional runs , the highway speed efficiency strategy has more impact when your weekly driving includes long highway segments. A 300 km return run to a regional centre every week is where the 100 vs 120 km/h conversation really matters financially

•       Remote driving , fuel prices at isolated rural outlets can be significantly higher than capital city prices, making efficiency on that stretch of driving more valuable than at home. Getting your car in optimal condition before a remote trip is good insurance

The Bottom Line

If your car is wasting fuel, it's almost certainly doing it in one of the areas covered in this guide. The good news is that the majority of the checks here are free or inexpensive, and most of the fixes are straightforward.

Tyre pressure alone can recover 2–5%. A dirty air filter can account for 6–11%. Smooth driving in the city is worth 10–20%. Buying on the right day of the weekly cycle saves $240–$360 per year without changing anything about how you drive.

You don't have to act on everything at once. Work through the checklist in order , starting with the free and fast checks at the top. Each item you address returns money to your pocket that the car was previously sending to the bowser.

Frequently Asked Questions

How do I calculate my car's actual fuel consumption?

The most accurate method is the full-tank method. Fill the tank completely to the first automatic cutoff of the nozzle , not topped up manually beyond that. Reset your trip odometer to zero. Drive normally for a full tank. Fill again at the same station if possible, noting exactly how many litres are required to bring the tank back to full. Divide that number of litres by the kilometres travelled on your trip odometer, then multiply by 100. This gives you L/100km. Repeat across two or three consecutive tanks to get a reliable average , a single tank can be skewed by unusual driving conditions. Compare your result to the manufacturer's stated combined cycle consumption (found in the owner's manual). Real-world consumption is typically 10–20% above the stated figure; if you're 25% or more above, there are likely specific factors contributing to the excess.

Can a dirty engine reduce fuel economy?

Yes , though it depends on what 'dirty' means in context. Internal engine deposits , carbon buildup on intake valves (particularly common in direct-injection GDI engines), injector deposits, and combustion chamber buildup , can affect how cleanly the engine burns its charge. GDI engines are particularly prone to intake valve carbon deposits because fuel is injected directly into the cylinder rather than through the intake port, meaning the fuel no longer washes the valves clean on each cycle. These deposits can affect airflow into the cylinder and combustion quality. Professional carbon cleaning , either chemical (fuel system cleaner through a specific process) or walnut blasting for intake valves , can recover modest economy gains in affected vehicles. For most standard maintenance purposes, using quality branded petrol with good detergent additives is the best routine prevention.

Is it worth buying a fuel-efficient tyre to save money?

Yes , tyre rolling resistance varies significantly between tyre models and compounds, and this difference shows up in real fuel consumption. Low rolling resistance (LRR) tyres are specifically engineered with compounds and constructions that flex less during rotation, reducing the energy absorbed as heat. The fuel economy difference between a standard tyre and a quality LRR tyre on the same vehicle has been measured at 2–5% in standardised testing. Over 15,000 km per year at $2.00/L and 9L/100km, a 3% economy improvement saves approximately $80 per year in fuel , which over the life of a tyre set adds up to a meaningful sum. When replacing tyres, it's worth comparing the rolling resistance rating (shown on the EU tyre label system) alongside price. A quality LRR tyre from a reputable brand may have a slightly higher purchase price but delivers lower running costs over its life.