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Why Is My Car Using More Gas All Of A Sudden? (12 Reasons Reviewed By Mechanic)

Why Is My Car Using More Gas All Of A Sudden? (12 Reasons Reviewed By Mechanic)
Fact Checked and Reviewed by: Kris Jackson, ASE-Certified Mechanic
Kris Jackson has been a mechanic since 2010 after graduating from UTI. He’s worked with several master mechanics and holds several ASE Certifications. You can read more about Kris here.

Your car is chugging gas like there’s no tomorrow, and you’re left wondering why. It’s as frustrating as seeing your smartphone battery plummet when you need it most.

Except, in this case, it’s costing you cash at the pump and that’s enough to get anyone’s attention. 

So, what’s the deal? Why is your car suddenly using more gas?

The most common culprit is a change in your driving habits, like sitting in more traffic or running quick errands which leads to short trips. However, reduced tire pressure (which can happen quickly during season changes), dirty air filters, bad spark plugs, and even misaligned wheels after a pothole can cause a sudden reduction in fuel efficiency. 

That’s the quick answer and covers some of the more likely explanations. But we’re going to cover a lot more possibilities and help you figure out which one makes the most sense for your situation. 

Let’s get started!

Reason 1: Change In Driving Habits

One of the most likely reasons your car is suddenly guzzling more gas is a change in your driving habits. Have you been idling in traffic more than usual? Running quick errands where your car doesn’t reach optimal operating temperature can also suck up fuel.

To identify this, ask yourself if your commute has become congested or if you’ve changed your daily driving routine. Did you start driving your kids to school, thereby adding frequent stops to your drive? All these factors can make your car’s computer adjust to non-ideal conditions, often resulting in lower fuel efficiency.

The impact can be significant. Studies show that idling can use a quarter to a half gallon of fuel per hour, depending on your car’s engine size and the air conditioner’s use. So, if you find yourself suddenly stuck in traffic for an hour daily, you’re looking at potentially wasting up to 3.5 gallons per week just idling. That translates to an additional weekly fuel cost of $10 to $15 for the average driver.

Another driving habit that can affect fuel economy is sudden acceleration or what some call “jackrabbit starts.” This might always be your habit, but if something changes in your morning routine (like a different start time with less traffic) you may be able to hit the throttle a little harder. Plus if it’s a family car, or you’re not the only one driving it then different driving styles might be responsible.

Driving at high speeds can also be a gas-guzzler. Again, a change in your route is usually required here but for every 5 mph you drive over 50 mph, you’re essentially paying about 19 cents more per gallon for gas, according to the EPA

Reason 2: Reduced Tire Pressure

If you’ve noticed a sudden drop in your fuel efficiency, another key suspect is reduced tire pressure.

Now, what can cause this to happen abruptly or suddenly?

One common culprit is a seasonal change, especially the transition from warm to cold weather. Tire pressure can drop about 1 psi for every 10°F decrease in temperature. So, as you go from summer to fall or winter, don’t be surprised if your TPMS (tire pressure monitoring system) alert pops up on your dashboard.

The impact can be significant. According to the U.S. Department of Energy, for every 1 psi drop in the pressure of all four tires, your gas mileage could deteriorate by as much as 0.2%. It might not sound like much, but these numbers add up over time, especially if you do a lot of driving.

The TPMS technology became standard in most vehicles around 2008, so if your car is newer than that, keep an eye on the dashboard for this alert. If you’re driving something older, you’ll need to stick with the old fashioned technique of using a tire pressure gauge- just be more alert during seasonal changes. 

Reason 3: Increased A/C Usage

Increased A/C usage during summer can lead to what feels like a sudden drop in fuel efficiency. It can vary between vehicles, but our research found that you can expect your fuel economy to be reduced by 1 to 4 mpg when you’re cranking the A/C

Reason 4: Extra Cargo 

Another often-overlooked factor that can dip into your fuel efficiency is increased vehicle load. We’re talking about heavy items like toolboxes, golf clubs, or that drum set you’re lugging around in the trunk. The more weight your car carries, the harder the engine has to work to move it, burning more fuel in the process.

I know, this might seem like a stretch at first. After all, how can this happen suddenly if you’re the one putting gear in your car?

The reason this feels sudden is because of the impact that hauling an extra load can have. Every extra 100 pounds can reduce your miles per gallon (MPG) by about 1-2%. So if your car usually gets 30 MPG, an additional 300 pounds could drop it down to approximately 28 MPG. That could be a few extra passengers or extra gear but either way it’s enough to notice over time. 

Then there’s roof racks, bike mounts, or even a loaded-up ski box that create additional drag, disrupting the aerodynamic flow around the car. This forces the engine to work harder, sapping even more fuel.

Then when you combine these two factors by adding an extra storage box and filling it with stuff, you can end up with a noticeable, and sudden, change in how much fuel your vehicle uses. 

Reason 5: Wheel Alignment Could Be Off

Bad alignment doesn’t just lead to uneven tire wear; it can also cause your engine to burn more fuel. According to Consumer Reports, “studies show that a 10 percent drop in rolling resistance equates to about a 1 percent improvement in fuel economy.”

Misalignment often results in one side of the tire wearing down faster than the other. This uneven wear, in turn, leads to an irregular tire surface, which increases rolling resistance. When this happens, the engine must use more energy to push the vehicle forward, reducing fuel efficiency.

So, can this misalignment happen suddenly?

In a way, yes. If you happen to hit a pothole or brush up against a curb, it can introduce an alignment issue. While a significant misalignment might be noticeable in your car’s handling, minor ones could slowly deteriorate fuel efficiency until you suddenly take note of the increased consumption.

Reason 6: Bad Spark Plugs

Spark plugs are the little guys responsible for igniting the air-fuel mixture in your engine. When they’re in good shape, your engine runs smoothly and your fuel economy is optimized.

But when they start to fail, they don’t ignite the air-fuel mixture in the cylinders as efficiently as they should. This means that not all of the fuel gets burned, resulting in incomplete combustion. When combustion is incomplete, the engine has to work harder to produce the same amount of power, using more fuel in the process.

This can have a big impact on fuel efficiency and while you’ll see stats like a 30% reduction in fuel efficiency floating around the internet, that’s definitely on the high end.

Still, it’s usually enough for you to notice and while spark plugs usually fail slowly, once a spark plug starts to fail, the degradation can compound fairly rapidly, meaning you’ll suddenly feel the effect one day even if the problem has been brewing for a while.

So, while the wear occurs over time, the perceptible impact can seem quite sudden. 

Spark plugs are generally located on the engine’s cylinder head and are easily accessible for most engines. They should typically be replaced every 30,000 to 90,000 miles depending on your car model and the type of spark plugs it uses. However, this range can vary, so consult your owner’s manual for specifics.

Reason 6: Issues With Brakes (Dragging Calipers And Worn Brake Pads)

When it comes to bad brakes, safety should be a bigger concern than spending extra cash at the pump! There are two common ways that brake problems can lead to a drop in MPG: dragging calipers and worn-out brake pads. 

Dragging Calipers

The caliper is a component that squeezes the brake pads against the brake rotor to slow down or stop your vehicle. Over time, or due to certain circumstances, calipers can get “stuck” or fail to release completely. This means that even when you’re not pressing on the brake, the brake pad remains in slight contact with the rotor.

Imagine trying to drive with the brakes partially engaged all the time—it’s not hard to see how this can reduce fuel efficiency. Not only will this drop your MPG, but you’ll also end up wearing out the brake caliper even faster, and the brake pads too since they’re always applied. It could also cause the rotors to become heat-saturated and turn blue (bluing rotors.)

Besides a sudden drop in fuel efficiency, you may notice changes like more sluggish acceleration, a hot wheel hub after even short trips, or the smell of hot brakes (smells like burning metal or rubber).   

As far as this occurring suddenly, hitting a really deep pothole, and hopping a curb, could cause the brake to constantly apply, as well as a failure of the caliper piston seal could have the potential to lead to a sticking caliper. Something lodging into the brake pads can lead to the parking brake seizing from bad hardware or cable.

Worn Out Brake Pads

Now, let’s talk about worn brake pads. While it might seem counterintuitive, having overly worn brake pads can contribute to fuel inefficiency in a roundabout way. Worn-out pads can lead to metal-on-metal contact, which results in increased resistance. The more resistance your engine has to work against, the more fuel it consumes.

How does this fit into the “suddenly” aspect?

Brake issues like dragging calipers or worn brake pads won’t happen overnight. However, the impact on fuel efficiency might only be noticed once it’s reached a critical point. For example, you might have had a caliper slowly sticking more over time, but only feel the reduction in fuel efficiency when it begins dragging considerably.

It’s like wearing shoes that are gradually wearing out; you might not notice any discomfort until a hole suddenly appears. Similarly, you might not realize your brakes are causing a decrease in fuel efficiency until the problem has significantly progressed.

Reason 7: Fuel Leaks 

A fuel leak isn’t just a fire hazard—it’s also a surefire way to watch your gas mileage plummet. Obviously, fuel efficiency shouldn’t be the biggest issue if your ride is leaking fuel though. 

Leaks can occur in various parts of the fuel system, from the tank itself to the fuel lines or even at the injectors. But the most common spots are the fuel lines, connectors, and the fuel tank’s underside, especially near the seams. 

If there is a leak, that’s big enough to drop your MPG, you’ll likely also notice a strong smell of gasoline- especially when the vehicle is running or shortly after you’ve parked. Another telltale sign is spotting a puddle of liquid—often with a rainbow sheen—under your vehicle.

Remember, gasoline evaporates quickly, so you might not always see puddles, but the smell can be a lingering clue.

Fuel leaks can certainly arise suddenly. A minor accident, driving over a sharp object, or sudden corrosion can breach the fuel system, leading to a leak. 

Reason 8: Faulty Sensors (O2 And Mass Air Flow)

Every modern car is equipped with an array of sensors designed to monitor and regulate the vehicle’s performance.

Two sensors vital to your car’s fuel efficiency are the Oxygen (O2) sensor and the Mass Air Flow (MAF) sensor. When these go awry, your fuel consumption can take an unexpected hit.

Oxygen (O2) Sensor:

Oxygen (O2) Sensor: This sensor measures the amount of unburned oxygen in the exhaust. It helps the car’s computer determine the optimal air-fuel ratio for combustion. When it malfunctions, your car might run either too rich (too much fuel) or too lean (too little fuel). Both situations can lead to poor gas mileage.

A failing O2 sensor may cause your check engine light to come on, and you might experience rough idling or decreased power but these little sensors can cause a long list of other problems as well

Mass Air Flow (MAF) Sensor: 

Positioned near the air filter, this sensor measures the amount of air entering the engine. The car’s computer uses this data to determine how much fuel to inject. A faulty MAF sensor can misreport the amount of air coming in, leading to an incorrect air-fuel mixture. Symptoms of a bad MAF include hesitation during acceleration, rough idling, stalling, and of course, reduced fuel efficiency.

Can These Sensors Cause A Sudden Drop In MPG? 

The tricky thing about sensors is that their deterioration can be gradual, but the effects become suddenly noticeable when they reach a tipping point. While wear and tear are natural, certain factors like contaminated air or excessive oil residues can cause these sensors to fail earlier than expected.

If you’re experiencing a sudden decrease in fuel economy accompanied by any of the above symptoms or a lit check engine light, a faulty sensor could be your culprit. Getting them checked and replaced if necessary can help restore your car’s fuel efficiency to its optimal level.

It’s also worth mentioning that there are other sensors that can also throw off the fuel trims and they may need to be checked out, like the coolant temp sensor, intake psi sensor (MAP), position sensors, throttle, basically, the whole network of sensors.

Reason 9: Fuel Injectors Are Dirty

Fuel injectors play a pivotal role in your car’s performance, acting as nozzles that spray fuel into the combustion chamber. As with any nozzle, from your sink to the end of your toothpaste tube, if debris accumulates around the exit, fluid won’t flow as efficiently. 

The same happens with fuel injectors. Contaminants can hinder the optimal mist of fuel, leading to incomplete combustion. In most cases, this doesn’t happen overnight but if you’re driving in especially dusty or dirty areas it can happen faster than you’d expect. 

According to ASE-certified Mechanic and contributor, Kris Jackson, “In this case it’s more likely that your air filter will get extremely clogged.” He also adds, “However, using a bad batch of fuel would be more likely to clog up the fuel injectors quickly.”

The impact on your fuel efficiency will be small (likely around 5%) but still enough to notice- especially after a few fill-ups. 

Cleaning them isn’t too complex. There are fuel additives on the market that can help clean the injectors, but for severe cases, a professional cleaning might be in order. 

Reason 10: Wrong Oil Viscosity 

When we talk about oil viscosity, we’re diving into how thick or thin that oil is. It’s akin to choosing between whole milk and skimmed milk, with both having distinct textures and flow rates. In the context of your car’s engine, the oil’s job is crucial: it’s the lubricant ensuring everything runs without too much friction.

If your ride has inadvertently ended up with oil that’s too thick for your vehicle, it won’t flow as seamlessly. Your engine has to work extra, akin to trying to sip a thick milkshake through a narrow straw. This added effort from your engine leads to burning more fuel.

On the contrary, using oil that’s too thin may not provide enough cushioning for engine parts, causing it to overcompensate in other areas, again consuming more gas. At the end of the day, it’s best to just stick to what the engineers want and not try to deviate from that just to get better gas mileage.

The impact of this on fuel efficiency is typically (expect less than 5% in most cases) but can vary a lot based on how off the oil selection is from the recommended viscosity. However, it certainly checks the “suddenly” box since it just takes one oil change to notice this difference. 

Reason 11: Exhaust System Issues

Fuel efficiency isn’t just about what goes into your engine – it’s also about what comes out. The exhaust system, responsible for removing waste gases, can be a culprit in sudden fuel consumption spikes.

If your car can’t expel waste gases effectively, it makes the engine work harder, which means more gas gets burned. Issues might be caused by clogged catalytic converters or a damaged muffler, among other things.

However, the big difference between this explanation and most others on the list is that exhaust problems are rarely subtle. You’ll hear it if your exhaust system is suddenly malfunctioning. For a great example of this, check out this Prius that sounds like it’s at the starting line of the Daytona 500 after its catalytic converter got jacked: 

Reason 12: Bad Clutch

This one is less likely (which is why it’s at the end of our list) but if you’re driving a manual then it’s worth considering. The clutch connects your engine to the transmission system. When it’s failing, efficient power transfer suffers, resulting in the car burning more fuel.

A common sign of a bad clutch is slipping when the clutch isn’t engaging and disengaging correctly. You might notice the engine revving higher without a corresponding increase in speed. This mismatch in engine effort and car performance can lead to higher fuel consumption.

A Dirty Air Filter Probably Isn’t To Blame

Whether you’re talking about a sudden drop or a gradual decline in your fuel efficiency, a dirty air filter might be one of the explanations you’ll hear. 

But this idea is based on carbureted engines and not the modern fuel-injection engines that almost entirely replaced carbureted engines by the mid-1990s. 

The U.S. Department of Energy confirmed this with a 2009 study showing that a dirty air filter doesn’t negatively affect gas mileage for cars with fuel-injection technology.

However, ASE-certified mechanic, and contributor, Kris Jackson isn’t convinced by this study. He states, “While fuel-injected cars can compensate by taking away fuel since it’s running richer but only so much before it affects the combustion correctly.”

That said, changing your air filter isn’t just about fuel efficiency. An overly clogged filter can let harmful particles into the engine, potentially causing damage over time. Plus, maintaining optimal airflow ensures your engine performs consistently, giving you a smoother driving experience.

Closing Thoughts

Many of these changes to fuel efficiency really are sudden. Adding a roof rack, filling your trunk with all the golf clubs, cranking the A/C up, or deciding to floor it at every light can make an instant impact. 

But many underlying issues don’t just spring up overnight. Spark plugs don’t degrade in a day and neither do brakes or fuel injectors. Instead, you suddenly feel their impact as they cross a certain threshold of “going bad”. 

That means more sudden causes can actually be caught, and prevented, with regular maintenance. Whether you’re doing it on your own or asking for an inspection the next time you get your oil changed (just make sure they use the right viscosity) regular care can save you money and time down the line. 

Ride safe!

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