Why Your Car Is Using More Petrol Than Usual

Most people notice it gradually. You realise you're filling up more often than usual, or the fuel gauge is dropping faster than it used to. The car hasn't broken down. The engine sounds fine. But something has clearly changed , and you're spending more at the bowser each week without an obvious explanation.

Increased fuel consumption is one of those problems that gets easy to ignore because nothing seems 'wrong' in the traditional sense. The car starts, it drives, it stops. But underneath that apparent normality, there are often very specific and very fixable reasons why economy has degraded , and tracking them down can save you a meaningful amount of money once prices per litre are what they are right now.

This guide covers every significant cause of increased fuel consumption, from the simple and free to fix through to the mechanical issues that need a professional. Most of the causes here are common, diagnosable, and cheaper to address than the ongoing cost of ignoring them.

First: Make Sure the Economy Has Actually Dropped

Before diving into causes, it's worth confirming that the economy actually has changed rather than just seeming like it has. Perception of fuel consumption can be misleading.

The most reliable way to track your real-world consumption is to:

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

•       Reset your trip odometer to zero

•       Drive normally until the next fill-up

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

•       Divide the litres used by the kilometres travelled and multiply by 100 , this gives you L/100km

Do this across two or three fill-ups to get a reliable baseline. Compare it to your manufacturer's stated combined economy figure (in your owner's manual) and to what you were achieving previously if you have that data.

One important caveat: economy varies significantly with how and where you drive. More city driving, more short trips, hotter weather requiring more AC, heavy traffic , all of these increase consumption without anything being wrong with the car. If your driving pattern has genuinely changed alongside the consumption increase, that may be the whole explanation.

Tyre-Related Causes , Often the First Place to Check

Under-Inflated Tyres

This is the single most common and most overlooked cause of increased fuel consumption in Australian passenger vehicles. Under-inflated tyres have a larger contact patch with the road and flex more during rotation, creating greater rolling resistance. The engine has to work harder to maintain speed, which means more fuel used for the same distance.

Research consistently shows that tyres running 10% below their recommended pressure increase rolling resistance by approximately 3%, translating directly to 2–3% more fuel per kilometre. At 20% below recommended pressure , which is entirely possible in neglected tyres , the penalty can reach 5–7%.

•       Check all four tyres including the spare when cold (before driving or after less than 3 km)

•       Find the correct pressure on the tyre placard inside the driver's door frame , not on the tyre sidewall, which shows maximum pressure

•       Check monthly , tyres naturally lose 1–2 PSI per month through normal permeation

•       Also check for uneven pressure between tyres , one noticeably lower than the others suggests a slow puncture or valve issue

Wheel Alignment Out of Specification

If your wheels are not properly aligned , pointing in slightly the wrong direction , the tyres scrub against the road rather than rolling freely. This creates significant rolling resistance that saps both fuel economy and tyre life simultaneously.

Signs that alignment may be off: the car pulls to one side, the steering wheel isn't centred when driving straight, or you notice uneven wear across the tyre (more worn on the inner or outer edge than the centre). A wheel alignment costs $60–$90 at most tyre shops and is worth doing if you notice any of these signs.

Wrong Tyre Type or Size

If your tyres have been replaced recently and a different size or specification was fitted, this can affect fuel economy. Wider tyres and high-performance 'sticky' compound tyres have higher rolling resistance than standard touring tyres and will increase consumption. Check that the replacement tyres match the size and load/speed rating specified in the owner's manual.

Maintenance Issues That Drain Economy

Dirty or Clogged Air Filter

The air filter prevents dirt and debris from entering the engine. Over time, it accumulates particles and becomes progressively more restrictive. A clogged air filter reduces the volume and flow of air entering the engine, forcing the fuel management system to compensate with richer fuel mixtures , more fuel per combustion event.

A significantly dirty air filter can reduce fuel economy by 6–10%, which on a car that normally does 8L/100km means it's doing 8.5–8.8L/100km instead. Over 15,000 km per year, that's 75–120 extra litres consumed , several hundred dollars' worth at current prices.

•       Air filters should be inspected at every service and replaced when dirty , typically every 20,000–40,000 km depending on conditions

•       In dusty conditions , outback driving, dirt roads, dusty worksites , filters clog much faster and should be checked every 10,000 km or sooner

•       Replacement cost is typically $20–$40 in parts, often done as part of a standard service

Worn or Fouled Spark Plugs

Spark plugs ignite the air-fuel mixture in each cylinder. When spark plugs are worn, their electrode gap has increased and the spark becomes weaker. When they're fouled with deposits, the spark may be inconsistent or may misfire entirely.

A weak or missing spark means incomplete combustion , the fuel doesn't fully burn, energy is wasted, and the engine compensates by using more fuel to maintain power output. Misfiring cylinders also cause rough running, increased emissions, and in severe cases, damage to catalytic converters.

•       Most modern vehicles have service intervals of 60,000–100,000 km for iridium or platinum spark plugs

•       Standard copper plugs in older vehicles should be replaced every 25,000–40,000 km

•       If your car is rough at idle, hesitates on acceleration, or has a noticeable misfire, spark plugs are a priority check

Degraded Engine Oil

Engine oil's viscosity changes as it ages and accumulates contaminants. Older oil becomes thicker, more viscous, and harder to pump through the engine's internal galleries. The additional internal friction this creates means the engine uses more energy , and therefore more fuel , to overcome it.

This effect is gradual and hard to notice day-to-day, but an engine running on oil that's 5,000 km overdue for a change may have measurably worse economy than the same engine on fresh oil. The improvement from a full oil service with the correct grade is typically 1–3% in fuel economy.

Blocked or Partially Clogged Fuel Injectors

Modern fuel injection systems deliver precisely metered amounts of fuel directly into each cylinder. Over time, carbon deposits from combustion accumulate on the injector nozzles, affecting the spray pattern and the volume of fuel delivered per injection event.

Clogged injectors either deliver too much fuel (rich running) or spray it in a poor pattern that doesn't atomise correctly , both waste fuel. The engine's management system may compensate by adjusting fuelling upward, increasing consumption further.

•       Symptoms: rough idle, hesitation on acceleration, poor cold start performance, increased consumption

•       Diagnosis: a mechanic can perform an injector flow test or use fuel system cleaner through the injectors

•       Treatment: professional injector cleaning ($100–$250) or in severe cases, injector replacement

Failing Oxygen Sensor

The oxygen sensor (also called a lambda sensor) monitors the oxygen content of the exhaust and feeds this data back to the engine management system, which uses it to maintain the correct air-fuel mixture. When this sensor fails or provides inaccurate readings, the engine may run with an excessively rich mixture , too much fuel relative to air , wasting fuel and increasing consumption.

A faulty oxygen sensor is one of the most common causes of a check engine light illumination, and it's also one of the most common causes of sudden, unexplained fuel economy drops of 10–20%.

•       If your check engine light is on alongside worsening economy, an oxygen sensor fault is high on the diagnosis list

•       Replacement cost: typically $150–$400 fitted depending on vehicle and sensor location

Faulty Thermostat

The engine thermostat regulates coolant temperature, keeping the engine in its optimal operating temperature range. A thermostat stuck in the open position allows coolant to circulate continuously, preventing the engine from reaching full operating temperature.

A cold-running engine has higher internal friction, runs with a richer fuel mixture (cold engines need more fuel), and doesn't operate at peak efficiency. If you notice your temperature gauge never reaching its normal position, or the heater not getting properly warm, a stuck-open thermostat may be the culprit , and it's a relatively inexpensive fix.

Dragging Brakes

If a brake calliper has seized or is partially dragging, the brake pad is in constant light contact with the rotor. This creates continuous mechanical resistance , the engine constantly has to work against braking force to maintain speed. The effect is similar to driving with the handbrake slightly on.

Signs of a dragging brake include a burning smell, unusual heat from one wheel after driving, the car pulling to one side, or uneven wear between front and rear brakes. A sticking calliper requires immediate attention , it's a safety issue as well as an economy one.

Driving and Usage Changes That Increase Consumption

More Short Trips and Cold Starts

Every cold start uses significantly more fuel than a warm-engine start. Modern fuel injection systems enrich the mixture for the first several minutes of operation while the engine, catalytic converter, and oxygen sensor reach operating temperature. A 3 km trip uses proportionally far more fuel per kilometre than a 30 km trip because the cold-start enrichment period represents a much larger percentage of the total distance.

If you've recently changed jobs, moved house, or shifted your routine to include more short trips, this alone can explain a noticeable economy decline without anything mechanical changing.

More Stop-Start City Driving

Highway driving and stop-start city driving are completely different in terms of economy. A car that returns 7L/100km on the highway might easily use 12–14L/100km in dense inner-city traffic with frequent braking and acceleration. If your regular routes have become more congested , new construction, changed traffic patterns, new routes , economy will have changed to reflect this.

Increased Vehicle Loads or Towing

Added weight requires more energy to accelerate and maintain speed. Every extra 100 kg of load increases fuel consumption by approximately 5% in city conditions. If you've been regularly carrying passengers, heavy loads in the ute tray, or towing, this will show up in your consumption figures.

Seasonal Changes , Heat and AC Load

Summer in Australia means air conditioning runs harder and longer. AC can add 5–15% to fuel consumption depending on ambient temperature, the vehicle's AC system, and driving conditions. If your economy declined as summer arrived and recovers in winter, air conditioning load is likely the primary explanation.

Seasonal Changes , Cold Starts and Winter Running

On the flip side, winter cold starts in alpine or southern areas of Australia cause richer running until operating temperature is reached. If most trips are short commutes in cold weather, economy can decline noticeably compared to summer's warmer start conditions.

Quick Diagnosis Guide: What Symptom Points Where

Symptom	Most Likely Cause(s)	Action
Economy dropped suddenly with no other symptoms	Oxygen sensor, air filter, tyre pressure	Check tyres first; scan for fault codes
Economy declined gradually over time	Spark plugs, injectors, degraded oil	Service overdue , start there
Check engine light on + worse economy	Oxygen sensor, misfire, injector fault	Get a diagnostic scan immediately
Rough idle + worse economy	Spark plugs, injectors, air filter, vacuum leak	Full service plus injector check
Car pulls to one side + worse economy	Wheel alignment, dragging brake calliper	Alignment check + brake inspection
Temperature gauge low, heater weak + worse economy	Faulty thermostat (stuck open)	Thermostat replacement
Burning smell from wheels after driving	Dragging brake calliper	Brake inspection urgently
Economy worse in summer only	Air conditioning load	Normal seasonal variation
Economy worse since new tyres fitted	Wrong tyre type, size or rolling resistance	Check tyre specification matches recommendation

Less Obvious Causes Worth Knowing About

Positive Crankcase Ventilation (PCV) Valve Failure

The PCV valve manages the pressure inside the engine's crankcase by routing blow-by gases back into the intake manifold. When this valve fails, it can cause excessive oil consumption and alter the air-fuel mixture entering the engine. It's a small, inexpensive component that's often overlooked in servicing but can contribute to economy issues in higher-mileage vehicles.

Catalytic Converter Blockage

A partially blocked catalytic converter creates back pressure in the exhaust system, making it harder for the engine to expel exhaust gases. This reduces engine efficiency and increases fuel consumption. A completely failed catalytic converter will typically set a check engine light and cause significant performance loss alongside economy deterioration.

Fuel Pressure Regulator Issues

The fuel pressure regulator maintains the correct pressure in the fuel rail feeding the injectors. If it fails and allows pressure to drop, injectors don't spray fuel atomise correctly, reducing combustion efficiency. If it fails in the other direction and allows excessive pressure, the engine runs rich. Either way, economy is affected.

Engine Compression Issues

In high-mileage engines, worn piston rings or cylinder walls can allow combustion gases to escape past the pistons. This reduces the efficiency of each combustion event , you're getting less work out of the same amount of fuel. Symptoms typically include blue smoke from the exhaust, increased oil consumption, and reduced power alongside worse economy. A compression test can confirm this.

Ethanol Blend Variation

If you recently switched from standard 91 unleaded to E10 (91 RON with up to 10% ethanol), this alone accounts for a 1–3% reduction in fuel economy. Ethanol has lower energy content per litre than pure petrol. If you've switched brands or the servo you use has changed their supply blend, this minor economy shift can appear confusing when it's actually just the composition of the fuel changing.

When to See a Mechanic and What to Tell Them

Not every economy issue needs a mechanic , under-inflated tyres, a dirty air filter, or an overdue oil service are things you can address yourself or at your next routine service. But some causes require professional diagnosis and repair.

Take the car to a mechanic if:

•       Your check engine or engine management light is illuminated , this indicates a stored fault code that a scan tool will read in minutes, pointing directly to the cause

•       You've noticed rough running, hesitation, misfiring, or unusual noises alongside the economy drop

•       The temperature gauge is running low or the heater isn't warming properly

•       You smell burning from around the wheels after driving , this is a safety issue, not just an economy one

•       The economy has dropped by more than 15% and you've already checked the obvious maintenance items

When you go to the mechanic, tell them:

•       How many litres per 100km you're currently using versus what you were getting previously

•       When the change became noticeable and whether it was sudden or gradual

•       Any other symptoms , rough idle, warning lights, unusual smells, handling changes

•       Your recent service history , when the oil was last changed, when spark plugs were last done

A good mechanic with this information can make a targeted diagnosis rather than a speculative one, which saves both time and money.

The Bottom Line

A car that's suddenly using more fuel than it used to is usually trying to tell you something. In most cases, the cause is maintenance-related , an overdue service, under-inflated tyres, a dirty air filter, or worn spark plugs , and the fix is straightforward and relatively cheap compared to the ongoing cost of the extra fuel being burned.

Start with the free and simple checks: tyre pressure, recent changes to your driving pattern, seasonal effects, and whether a service is overdue. If those don't explain it, move to the mechanical checks: air filter, spark plugs, oxygen sensor, injectors. If there's a warning light showing, get it scanned before anything else.

The cost of diagnosis and repair is almost always less than the cost of months or years of running an engine at reduced efficiency. At current prices, every unexplained 10% economy drop on a typical Aussie vehicle is costing $200–$300 extra per year , year after year until the cause is found and fixed.

Frequently Asked Questions

How much worse economy should I expect in summer compared to winter?

Air conditioning is the main driver of seasonal economy variation in Australian passenger vehicles. In moderate conditions, AC can increase fuel consumption by 5–8%. In extreme heat , 38–40°C days in Queensland, WA, NT, and SA , with maximum cooling demand, the impact can be 10–15% in city driving conditions. A car that returns 9L/100km in mild autumn weather might be using 10–10.5L/100km in peak summer with AC running constantly. This is normal and expected, not a sign of a problem. The flip side is that economy typically improves in autumn and spring when AC demand is lower , which can make a pre-existing mechanical issue harder to notice until the seasonal variation is stripped out.

Will a service fix my fuel economy?

In many cases, yes , particularly if you're overdue for a service. A standard service that includes fresh engine oil and filter, a new air filter, and fresh spark plugs addresses three of the most common maintenance-related causes of economy decline. If your economy has been gradually worsening over many thousands of kilometres and a service was overdue, the improvement from a full service can be noticeable , sometimes recovering 5–10% of economy that had been slowly lost. However, if a mechanical fault is causing the economy issue , a faulty oxygen sensor, a dragging brake calliper, a stuck thermostat , a service alone won't fix it. Those specific faults need diagnosis and targeted repair.

Could bad petrol cause my car to use more fuel?

True 'bad petrol' , genuinely contaminated or off-specification petrol from a reputable retailer , is rare in Australia. The ACCC and state fuel quality regulators conduct testing and the major retailers maintain quality controls. That said, there are some legitimate causes of economy variation related to the petrol itself. If you recently switched from standard 91 to E10 (ethanol blend), expect a 1–3% economy reduction because ethanol has lower energy density. If you used a lower octane grade than your engine requires and it's running with retarded ignition timing as a result, economy will be worse. And if you're buying from an unfamiliar or very cheap outlet, substandard fuel quality is possible, though uncommon with licensed retailers. Switching back to a major brand for a few tanks and monitoring whether economy recovers is a reasonable test if you suspect the petrol is a factor.