Why Is My Car Using So Much Fuel? Causes and What to Check
Most people do not notice fuel economy going bad. It does not fail like a flat tyre, it creeps. A few extra dollars a tank, the range on the dash reading a bit lower than it used to, and then one day you realise you are filling up noticeably more often than you were six months ago. By then the question is the one that lands in the app constantly: why is my car suddenly drinking fuel?
The honest answer is that it is usually not one big thing, it is one of a handful of common culprits, most of them cheap to check. The trick is working out whether you actually have a fault or whether the number just feels worse than it is. So start there.
First, work out if it is real
Before you chase a fault, get a real figure. The dash readout is handy but not gospel, and "it feels thirsty" is not a diagnosis.
Fill the tank, note the odometer, drive normally for a tank or two, then fill up again and do the sum: litres put in, divided by kilometres travelled, times 100. That gives you litres per 100km, the number that matters. Compare it to what your car is supposed to do. Every model has a rated combined figure, and while nobody hits the brochure number in the real world, you want to know if you are sitting reasonably close to it or way over.
If you are a little over the rated figure, that is often just real-world driving. If you are ten, twenty, thirty per cent over what you used to get in the same car on the same commute, something has changed, and that is worth chasing.
The common causes, roughly most likely first
Under-inflated tyres
This is the one everyone forgets and it makes a real difference. Soft tyres have more rolling resistance, so the engine works harder to keep you moving. Tyres lose pressure slowly over time even without a leak, so a set that has not been checked in a few months can easily be down enough to cost you fuel. It is free to check and the first thing to rule out. If yours keep going soft faster than that, that is a separate problem, and we cover it in why your tyre keeps losing pressure.
A lazy or dead oxygen sensor
This is one of the biggest hidden fuel wasters. The oxygen sensor in the exhaust tells the engine computer how much fuel to add. When it gets old and slow, or fails outright, the computer stops trusting it and defaults to dumping in more fuel than the engine needs, running rich. It often, but not always, brings on the check engine light. If your economy has dropped and the light is on, this is high on the list. A related lean-running fault throws its own code, and we break that one down in the P0171 system too lean code.
Worn spark plugs or tired ignition
If the plugs are worn out, combustion is not clean, and unburnt fuel is wasted energy. Plugs are a service item with a defined life, and running them well past it costs you both power and economy. Our guide on when to replace spark plugs covers the intervals and the symptoms.
A stuck-open thermostat
Here is a sneaky one. If the thermostat is stuck open, the engine never properly reaches operating temperature, so the computer keeps it in warm-up mode and runs it rich the whole time. The tell is that the temperature gauge sits low and never climbs to normal, and the cabin heater struggles to get hot. Cheap part, real economy hit.
Dragging brakes
If a brake caliper is sticking or a park brake is not fully releasing, the car is effectively driving with the brakes lightly on all the time. That is constant drag the engine has to overcome. Signs are a wheel that is much hotter than the others after a drive, a smell of hot brakes, or the car feeling like it is being held back.
A dirty air filter, but keep it in perspective
A blocked air filter gets blamed for everything, and it is worth checking because it is cheap, but on a modern fuel-injected engine the economy hit from a dirty filter is smaller than the myth suggests. It affects performance more than fuel use. Replace it if it is filthy, by all means, but do not expect it to be the silver bullet on its own.
Injectors running dirty or rich
Fuel injectors that are dirty, worn or leaking can upset the fuel mixture and push consumption up, especially on higher mileage engines. If you are also getting a fuel smell or rough running, that points here. We go deeper on that in leaking fuel injectors on high mileage engines.
The stuff that is just physics
Not every cause is a fault. Short trips where the engine never warms up, a lot of cold starts, roof racks or a loaded roof box adding drag, air conditioning working hard in summer, heavy loads, and a heavy right foot all genuinely raise consumption. If your driving has changed, your economy will too, and there is nothing to fix.
How to diagnose it yourself
Work through it in order and you will usually land on the cause without paying anyone:
- Get the real number. Litres per 100km over a couple of tanks, compared to your car's rated figure and to what you used to get.
- Check tyre pressures. Free, and a common culprit. Set them to the placard pressure on the door jamb.
- Look for a check engine light. An illuminated light, especially with a rich or lean code, points straight at a sensor like the oxygen or air-flow sensor. If the car has gone into a limp state as well, read why your car is in limp mode.
- Check the temperature gauge. If it never reaches normal, suspect the thermostat.
- Feel the wheels after a drive. One much hotter than the rest suggests a dragging brake.
- Review the service items. Spark plugs and air filter overdue? That is easy ground to recover.
- Note any rough running or fuel smell. That shifts suspicion toward injectors or the ignition side.
What it costs to sort
In tiers rather than fixed prices. Tyre pressures and an air filter are the cheap, do-it-yourself end. Spark plugs and a thermostat are a moderate part-and-labour job on most cars. An oxygen sensor or an air-flow sensor is a mid-range part that often pays for itself quickly in fuel saved once the engine stops running rich. Dragging brakes and injector work sit at the higher end. Knowing which of these you are dealing with, from the checks above, means you are not signing off on a big job when a twenty minute one would have fixed it.
Frequently asked questions
Can bad fuel economy trigger the check engine light? The underlying fault often can. A failing oxygen sensor, a dodgy air-flow sensor or a misfire will usually set a code and light the dash, and all of those also cost you fuel. Poor economy on its own with no light is more likely tyres, brakes, a thermostat or driving conditions.
Does premium fuel improve economy? Only if your car specifically requires or is tuned for higher octane. In an engine designed for regular unleaded, premium will not meaningfully improve economy, you are just paying more per litre. Use what the manufacturer specifies.
Why did my economy get worse in winter? Cold engines run richer until they warm up, cold air is denser, and short trips in the cold mean the engine spends more of its time not yet warmed up. A cold-weather dip is normal. A big or lasting one still deserves the checks above.
How much worse than the rated figure is normal? Nobody matches the brochure in the real world, and being modestly over the rated combined figure is expected, especially with city driving, air conditioning or a heavy load. It is a sudden or large jump against your own past figures in the same car that signals a fault.
Pin down where your fuel is going
Fuel economy is the classic case of many small causes wearing the same symptom, so the fix depends entirely on which one you have got. If you want to narrow it without guessing, that is what the TorqueBot AI car chat is for. Tell it your make, model and year, your actual litres per 100km, and whether anything else has changed like a warning light, a cold-running temperature gauge or a fuel smell, and it will work through the likely causes for your engine and point you at the cheapest thing to check first.