Why Is My Gas Mileage Going Down? (11 Reasons Why)

As more cars on the road reach high-mileage milestones, you may wonder, “Why is my car’s gas mileage getting worse?” After 100,000 miles, what began as an EPA-rated 28 miles per gallon car may end up being closer to a 20 MPG car—or even worse.

Fuel efficiency loss may not only drop your gas tank quicker but also your wallet. So, let’s delve more into deeper information about why your gas mileage is decreasing!

Why Is My Gas Mileage Going Down?

Your gas mileage may be going down because of malfunctioning or failing car components, including clogged fuel injectors and filters, old engine air filters, and dirty oxygen sensors. Also, damaged piston rings can cause your fuel efficiency to decrease. Further, faulty ignition systems, incorrect engine oil, and dirty mass air flow are common reasons your gas mileage drops.

Learn more about why your gas mileage is going down in this post. Here are 11 reasons why; stay tuned!

1. Clogged Or Malfunctioning Fuel Injectors

Dirty fuel injectors are among the most likely reasons for a decrease in fuel efficiency, which are the nozzles that spray fuel from every engine cylinder.

Further, the spray pattern of a fuel injector must be exact for it to mix with air and correctly combust within the engine.

Whenever a fuel injector has become dirty or blocked up, it can spray fuel inefficiently, similar to a low-pressure shower head.

Also, it can help minimize engine effectiveness and decrease fuel economy. In addition, you can often clean the fuel injector nozzles.

Therefore, if a poor spray pattern causes internal damage, the injectors may need to be replaced.

2. Old Engine Air Filter

To power vehicles, engines must draw in air.

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That said, if your engine air filter is highly clogged or dirty, your engine will be unable to “breathe,” and older engines will burn more fuel to achieve the same speed.

So, relatively new engines may operate poorly to adjust for a clogged air filter. This issue is especially prevalent in older cars that use carburetors.

Therefore, engine air filters must be replaced each 15,000 to 30,000 miles, but try to figure out your owner’s manual or have it thoroughly checked during the following oil change.

3. Filthy Oxygen Sensor

The most likely reason for lower gas mileage is faulty oxygen sensors. With that, O2 sensors detect how lean or rich the exhaust gases exiting your engine are.

So, a clogged sensor can end up causing your engine to combust too much fuel, reducing efficiency by up to 40%.

Therefore, you’ll suddenly experience more fuel loss than it should and have frequent trips to the gasoline station.

4. Clogged Fuel Filter

Fuel filters prevent pollutants in the fuel from traveling through the engine and bad fuel injectors and other critical components.

Of course, an obstructed fuel filter can reduce fuel pressure, causing your engine to perform poorly. Therefore, older vehicles must update their fuel filters every two years or 30,000 miles.

Also, bring your car to your local expert mechanic for a fuel pressure test if you seriously doubt a dirty fuel filter is causing your reduced gas mileage.

5. Damaged Piston Rings

To generate compression, the piston rings within your vehicle’s engine cylinders form a seal toward the cylinder walls.

Of course, when piston rings wear out, they can no longer form that seal, and the engine loses enough pressure. As a result, fuel economy is compromised.

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So, the engine oil lubricates the piston rings and improves fuel efficiency.

But, of course, the ideal solution is regularly changing your car’s oil with the recommended oil type listed in your manual.

6. Faulty Ignition System

Faulty Ignition System

The ignition system, which consists of spark plugs, coils, and wires, is in charge of burning the air-fuel mixture in your vehicle’s engine.

That said, if any of these components fails, it may cause the engine to misfire. Whenever the fuel in an engine cylinder does not ignite, this is referred to as a misfire.

So, because unburned fuel can’t power your vehicle, it wastes gas and reduces your fuel economy.

Therefore, if the ignition system is held responsible, you may notice rough idling, sputtering, or a general reduction in engine power.

7. Incorrect Engine Oil

There is a myth that older vehicles require thicker engine oil to avoid issues. However, thickened oil increases the resistance among engine parts, reducing fuel efficiency.

High mileage engine oils could aid in the reduction of oil leaking and usage. But, the best factor you can do to improve your car’s fuel efficiency is to change the oil regularly.

8. Filthy Mass Air Flow

Mass airflow sensors measure the quantity of air flowing into your vehicle’s engine.

Like an oxygen sensor, the mass airflow sensor transmits data to the computer, calculates the correct air-fuel percentage in the engine, and adapts fuel injection correspondingly.

On the other hand, a dirty airflow sensor will end up causing the car’s computer to miscount the proper air-fuel combination, causing reduced fuel efficiency and even engine stall.

Therefore, you should use a particular cleaning solution to clean mass airflow sensors.

9. Underinflated Tires

Low-pressure tires are one common source of decreased miles per gallon. With that, underinflated tires have higher rolling resistance on the highway and a lower specific diameter.

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In addition, there are car repair centers with tires with a low rolling resistance tread developed to save you funds and fuel.

Further, fuel-efficient tires contribute significantly to a cleaner environment and more cost-effective trips.

10. Damaged Or Stuck Brakes

It is not only risky to drive with damaged brake parts, but thick brakes may also lower your gas mileage.

For example, sticky brake pads or stuck calipers cause resistance to your car’s forward movement.

Brake drag occurs when your engine should actively battle with your brakes just to keep moving, resulting in a significant decrease in fuel efficiency.

11. Poor Alignment

If you have been driving with a steering wheel that wouldn’t sit directly for a while, your wheel alignment is probably resulting in additional costs at the pump.

That said, it is because misaligned wheels, like low-pressure tires, cause resistance to move straight down the road.

As a result, fighting this resistance always necessitates more effort from the engine and reduces fuel economy.

To know more, you can also read our posts on why your brake pedal is soft, why your brake caliper is sticking, and why your Bluetooth is not working in your car.


Sudden or mysterious falls in fuel efficiency indicate that it’s time to pop the hood. Even though it isn’t a critical issue, it is the first sign of a car component weakening or failing.

Do you want to restore your vehicle’s fuel economy to its previous car peaks?

If so, then you should perform regular maintenance and consult your expert mechanics once something wrong happens.

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