If your car has suddenly started using more fuel, possible causes of poor fuel economy include low tyre pressure, changes in driving conditions, overdue maintenance, engine or fuel-system problems, sensor faults, or extra resistance that makes the engine work harder.
A small change between tanks isn’t always a sign of a fault. But if you’re filling up noticeably more often without changing how or where you drive, it’s worth finding out what’s changed.
Sudden poor fuel economy can come from tyre pressure, driving conditions, overdue maintenance, or an underlying engine or mechanical problem.
You know roughly how long a tank normally lasts. Then suddenly, you’re back at the petrol station much sooner than expected.
One bad tank doesn’t necessarily mean something is wrong. Traffic, short trips, heavier loads, air-conditioning use and changes in driving style can all affect fuel consumption. But if the difference keeps happening without an obvious explanation, it’s reasonable to start looking at possible causes of poor fuel economy.
The important part is working out whether something about your driving has changed or whether the car itself is trying to tell you something.
Has Your Fuel Economy Actually Changed?
Before assuming there’s a mechanical problem, check whether the change is consistent.
Fuel gauge readings aren’t always a precise way to compare consumption. A better approach is to compare how many litres you’re using over a similar number of kilometres across several fills.
Think about what has changed as well. Have you been making more short trips? Sitting in heavier traffic? Carrying more weight? Using the air conditioning more often? These differences can affect how much fuel you use without there being anything mechanically wrong with the car.
If your normal driving hasn’t changed but fuel consumption clearly has, it’s time to look further.
What Can Suddenly Cause Poor Fuel Economy?
There isn’t one component responsible for fuel consumption. Your engine, tyres, fuel system, ignition system and even the amount of resistance the vehicle has to overcome can influence how efficiently it moves.
These are some of the areas worth considering when your usual fuel consumption suddenly changes.
Your Tyre Pressure Has Dropped
Underinflated tyres create additional rolling resistance. That means the engine has to work harder to move the vehicle, which can increase fuel consumption.
Queensland’s Department of Transport and Main Roads specifically notes that properly maintained tyres can help reduce fuel consumption. RACQ also recommends checking tyre pressure at least monthly and before longer trips.
Use the pressure recommended by your vehicle manufacturer rather than the maximum figure printed on the tyre sidewall. While you’re there, look for damage and uneven wear. Regular tyre maintenance and rotation can also help you spot unusual wear before it becomes more serious.
Your Driving Conditions Have Changed
Stop-start traffic usually requires more acceleration and braking than steady driving. Frequent short journeys can also affect your normal fuel-consumption pattern.
Think about whether your routine changed around the same time you noticed the problem. A new commute, heavier traffic, shorter journeys or carrying additional weight can all influence how quickly you go through a tank.
If everything about your driving is essentially the same and the increase is significant or persistent, however, a change in the vehicle itself becomes more likely.
Your Car Is Overdue for Maintenance
A car doesn’t suddenly become inefficient the day it passes its scheduled service interval. However, neglected maintenance can contribute to poorer performance over time.
Depending on the vehicle and its maintenance schedule, servicing can involve items such as engine oil and filters, spark plugs, fluids and other components that affect how well the vehicle runs. RACQ also lists regular servicing as an important part of reducing breakdown risk and maintaining the vehicle.
Knowing the correct car service intervals for your vehicle can help prevent routine maintenance from being forgotten until a noticeable problem appears.
The Engine Isn’t Running Properly
Poor fuel economy can sometimes appear alongside other changes in engine performance.
You might notice rough running, harder starting, reduced power, unusual noises, hesitation, or a dashboard warning light. The exact combination of symptoms can help narrow down where the problem lies.
For example, a car that hesitates when accelerating may have an issue affecting how the engine receives or burns fuel. Poor fuel economy alongside hesitation is therefore more useful diagnostically than fuel consumption alone.
A Sensor Isn’t Giving the Engine the Right Information
Modern engines use several sensors to help control fuel delivery, ignition and emissions.
If a relevant sensor begins sending incorrect information, the engine management system may not control the air-fuel mixture as intended. Depending on the fault, you may notice increased fuel consumption, changes in performance, rough running, or a warning light.
This is one reason replacing parts based only on poor fuel economy isn’t a good approach. Proper diagnosis can determine whether an electronic fault is present rather than guessing which component might be responsible.
Can a Check Engine Light Be Connected to Poor Fuel Economy?
It can be.
The check engine light can illuminate for many reasons, some of which may affect engine efficiency and fuel consumption. The light itself doesn’t tell you which component has failed, so diagnostic information is needed to understand what’s happening.
If fuel use has increased around the same time the light appeared, treat the two symptoms as potentially related until the cause is known. Whether it’s appropriate to keep driving with a check engine light on depends on the circumstances and how the vehicle is behaving.
A flashing warning light or a car that’s running very poorly deserves more immediate attention than a vehicle that otherwise appears to operate normally.
Can Bad Fuel Make Your Car Use More Petrol?
Fuel quality can affect how an engine runs, but don’t assume every sudden increase in consumption was caused by the last service station you visited.
Contaminated or unsuitable fuel can produce symptoms such as rough running, hesitation, difficult starting, or reduced performance depending on the situation. Using the fuel grade specified for your vehicle is also important.
If the change started immediately after refuelling and is accompanied by new engine symptoms, fuel quality is one possibility worth investigating. Bad fuel can cause engine problems, so repeated rough running or poor performance shouldn’t simply be ignored.
Could an Air Filter Affect Fuel Consumption?
An engine needs the correct amount of clean air for combustion, so the intake system is part of the overall picture when diagnosing performance problems.
However, don’t assume that replacing an air filter will automatically fix poor fuel economy. Modern engine-management systems can compensate for certain changes, and there are many other possible causes.
The better approach is to follow the manufacturer’s servicing schedule and diagnose the vehicle based on all of its symptoms rather than replacing maintenance items at random.
Can Brakes or Wheels Make the Engine Work Harder?
Yes. Fuel economy isn’t only about what happens inside the engine.
If a wheel doesn’t rotate as freely as it should, the engine may have to work harder to maintain speed. Brake drag, wheel or tyre problems, and certain mechanical faults can all create additional resistance.
You may notice other symptoms at the same time, such as the vehicle pulling, unusual heat around a wheel, changes in braking, or different handling.
Changes in braking behaviour shouldn’t be ignored regardless of fuel consumption. Keeping up with brake and suspension maintenance can help identify worn or abnormal components before they create further problems.
Does Air Conditioning Increase Fuel Consumption?
Using air conditioning requires energy, so it can affect fuel use.
How noticeable that difference is depends on the vehicle, conditions and how the system is being used. If you’ve recently been using the air conditioning much more because of hotter weather, it may explain part of a change in consumption.
But if fuel use has increased dramatically while your driving habits and air-conditioning use remain similar, don’t automatically blame the AC. Look at the overall pattern and any other symptoms the vehicle has developed.
Can Carrying Extra Weight Make a Difference?
Yes. The more weight a vehicle has to move, the more energy it generally requires.
A boot full of heavy equipment, extra passengers, or items that remain in the car unnecessarily can contribute to higher fuel consumption. Roof racks and roof-mounted cargo can also increase aerodynamic drag.
These factors are particularly useful to consider if the fuel economy changed after loading the vehicle differently. Removing unnecessary weight or external accessories may help you determine whether they’re contributing.
What If the Car Also Smells Like Fuel?
A noticeable fuel smell shouldn’t simply be treated as another fuel-economy problem.
If you can smell petrol or diesel strongly around the vehicle, particularly if you can see a leak, avoid treating it as something to monitor for a few more tanks. Fuel leaks can create a fire risk and need prompt attention.
Likewise, unexplained liquid underneath the vehicle deserves investigation. The appearance and location of a fluid leak under a car can provide clues, although an inspection may still be needed to identify the source accurately.
When Should Poor Fuel Economy Be Checked by a Mechanic?
A small variation between tanks doesn’t necessarily require a workshop visit. Fuel consumption naturally changes with traffic, weather, trip length, load and driving style.
It becomes more worth investigating when the change is significant, persists over several fills, or appears alongside another symptom.
Warning lights, hesitation, rough running, difficult starting, unusual noises, fuel smells, loss of power or changes in braking can all provide important clues. These kinds of mechanical warning signs can help distinguish an ordinary change in fuel use from a developing vehicle problem.
How Is Poor Fuel Economy Diagnosed?
There isn’t a single “fuel economy test” that identifies every possible cause.
Diagnosis starts with understanding what’s changed. A mechanic may ask when the problem began, how much consumption has increased, whether your driving pattern changed, and whether you’ve noticed warning lights or other symptoms.
From there, the vehicle can be checked based on the evidence. That might involve tyres, brakes, engine operation, maintenance items, electronic fault information, fuel delivery, ignition, or other systems depending on the symptoms.
This avoids replacing parts unnecessarily and gives you a better chance of addressing the actual cause.
Don’t Ignore a Fuel Bill That’s Suddenly Changed
There are plenty of everyday reasons your fuel consumption can move up or down. Traffic, short trips, tyre pressure, air-conditioning use, extra weight and driving conditions can all make a difference.
But when you’re filling up noticeably more often and nothing about your routine has changed, an underlying vehicle problem becomes worth considering. Finding the cause early may also reveal another maintenance or mechanical issue before it becomes more noticeable.
If your fuel consumption has suddenly increased or you’ve noticed changes in performance at the same time, contact us to have the vehicle checked and find out what’s causing the change.
Frequently Asked Questions
Why is my car suddenly using more fuel than normal?
Possible causes include underinflated tyres, changes in driving conditions, extra vehicle load, overdue maintenance, engine performance problems, electronic faults, or mechanical resistance. Compare several tanks and consider whether anything about your driving has changed.
Can low tyre pressure increase fuel consumption?
Yes. Underinflated tyres increase rolling resistance, which means the engine has to work harder to move the vehicle. Queensland TMR states that maintaining tyres properly can help reduce fuel consumption.
Can overdue servicing cause poor fuel economy?
Neglected maintenance can contribute to poor vehicle performance and efficiency. Follow the service schedule recommended for your particular vehicle rather than waiting until fuel consumption or performance changes.
Can a check engine light cause higher fuel consumption?
The warning light itself doesn’t increase fuel consumption, but the fault that triggered it may affect engine operation or efficiency. Diagnostic testing is needed to determine the actual cause.
Does air conditioning make a car use more fuel?
Air conditioning requires energy to operate and can increase fuel use. The size of the effect varies with the vehicle, weather, driving conditions and how the system is used.
Should I worry if my fuel economy suddenly gets worse?
One tank isn’t always enough to identify a problem. If consumption remains noticeably higher over several fills without an obvious change in driving conditions, or you also notice warning lights, poor performance, smells, leaks, or unusual noises, have the vehicle checked.
