Why Am I Getting Bad Gas Mileage: 12 Reasons Your MPG Is Suffering

A woman in a pink shirt filling up her vehicle's gas tank at the gas station.

Not all vehicle owners drive the same vehicle, but it’s safe to say every driver wants to get better gas mileage. If you’re reading this, it’s probably because you’re filling up your gas tank more than you used to, or maybe you’re wondering why your gas miles per gallon (MPG) dropped significantly on your last road trip. Whatever it is, we’re here to help. 

In this blog, we’ll answer the ever-pressing question of “why am I getting bad gas mileage?” by covering 12 of the top reasons your gas mileage is decreasing.

1. Old Engine Air Filter

A clogged air filter may negatively affect the performance of a vehicle’s engine. Just like people, vehicles need clean air to function properly. Your vehicle’s engine cannot “breathe” if the air filter is clogged or dirty, which in turn makes your vehicle work harder (i.e. expend more gasoline) to get you from point A to point B.

Typically, engine air filters need to be replaced every 15,000 to 30,000 miles, but we recommend checking your vehicle’s owner’s manual for exact specifications.

2. Clogged Fuel Filter

Does a clogged fuel filter affect gas mileage? Yes. Injectors and other critical engine components are protected from damage by fuel filters, which prevent contaminants from traveling throughout the engine. Having a clogged fuel filter can cause low fuel pressure and poor performance as contaminants accumulate, causing your vehicle’s fuel to become sludgy and difficult to process. 

We recommend having your vehicle’s fuel filter changed every 30,000 miles, or every two years in older vehicles, especially.

3. Worn Out Piston Rings

Compression is created when the piston rings in your engine cylinders seal against the cylinder walls. Wear and tear on piston rings prevents them from creating a seal, so the engine loses pressure, causing your vehicle to emit more fuel just to stay running.

A picture of a man replacing worn-out piston rings under the hood of a car.

4. Clogged Or Damaged Fuel Injectors

Driving with a bad fuel injector is a recipe for poor performance and wasted money. Imagine if your shower head wasn’t functioning properly. All that normal precision and force turns into a haphazard spray, and you have to use twice as much water just to get the normal effect. 

The same is true for your vehicle when it has clogged or damaged fuel injectors. In fact, one of the most common answers to the question of “why am I getting bad gas mileage?” is that you have a clogged fuel injector.

 Each engine cylinder has an injector that sprays fuel into it. For fuel to properly mix with air and combust within the engine, the spray pattern of the injector must be very precise. 

If a fuel injector becomes dirty or clogged, it will cease to spray fuel efficiently. Fuel injectors should be cleaned with a high-pressure cleaner. However, if the injectors are damaged internally and causing a poor spray pattern, they may need to be replaced completely.

5. Dirty Oxygen Sensor

Since 1996, all vehicles have used oxygen sensors in place of carburetors for their engines to receive the right amount of air and fuel for combustion. Oxygen sensors (O2 sensors) measure the amount of rich or lean gas that leaves your engine and send a signal to the computer to tell it how much fuel to add. 

According to Edmunds, dirty oxygen sensors may cause incorrect measurements, causing your engine to burn too much fuel, resulting in a 40% reduction in efficiency. 

The oxygen sensors in your vehicle may need to be replaced before 100,000 miles if your check engine light is on. Fortunately, O2 sensors are relatively inexpensive to replace, so you can save on fuel and keep your vehicle’s emissions under control.

6. Dirty Mass Airflow Sensor

Sensors that measure airflow into the engine and send data to the onboard computer are known as mass airflow sensors. This sensor calculates the correct ratio of air to fuel for the engine, and the computer adjusts fuel injection accordingly. 

Similar to a vehicle’s O2 sensor, a vehicle’s computer will miscalculate the proper air-fuel mixture when the airflow sensor is dirty. All of this means decreased fuel efficiency or even engine stalls. 

Your vehicle’s mass airflow sensor should be cleaned every six months. We recommend having it cleaned when you drop your vehicle off for an oil change.

7. Bad Ignition System Parts

Spark plugs, coils, and wires are a part of your vehicle’s ignition system. Once a vehicle’s ignition switch is activated, it triggers the voltage from the battery to the ignition coil to produce the engine spark. The engine spark from the coil(s) is directed to the spark plugs to ignite the fuel and make your vehicle run. If any of the parts of your ignition system malfunction, it can cause the engine to misfire.

 Fuel in a cylinder fails to burn when a misfire occurs, and as unburnt fuel cannot power your vehicle, it wastes gas and lowers your fuel economy. If the ignition system is malfunctioning, you may notice rough idling, stumbling, or a loss of power. Spark plugs are the most common cause of a failing ignition system.

A close-up picture of engine oil being poured into a vehicle's engine oil pan.

8. Old Or Incorrect Engine Oil

Older vehicles are often advised to use thicker engine oil to prevent leaks. In theory, thicker oil is less likely to leak through cracks in internal seals and gaskets as they age and become brittle. 

“High mileage engine oils” can accomplish this by using conditioning additives to help old seals become more flexible and improve performance. The thickness of the oil is also slightly increased to help worn piston rings seal better. The thicker the oil, however, the greater the resistance between engine parts, reducing fuel efficiency.

The right motor oil is essential to keep modern engines lubricated and protected. Regularly changing the oil in your vehicle according to the type specified in the owner’s manual will help you maintain optimum gas mileage. It will also help reduce oil leaks and consumption.

9. Underinflated Tires

The reason underinflated tires reduce MPG is because of something called a “high rolling resistance”. Tire rolling resistance is the energy that your vehicle needs to send to your tires to maintain movement at a consistent speed. In other words, it’s the effort required to keep a tire rolling.

In modern vehicles, tire pressure monitoring systems (TPMS) alert drivers when they need to fill up their tires, but the system only alerts a driver after the PSI drops significantly below the recommended pressure. Underinflated tires create drag and unnecessary friction between your vehicle’s tires and the road, which lowers fuel economy even if the TPMS doesn’t display a warning.

Regularly checking your tires’ pressure and adding air when necessary is an easy way to help you maintain fuel efficiency.

10. Worn Or Stuck Brakes

Still wondering why you’re getting bad gas mileage? Worn or stuck brakes may be the culprit. If the forward motion of your vehicle is hindered by a stuck brake caliper or sticky brake pads, your engine will be constantly fighting with the brakes just to move, thus reducing fuel economy.

11. Poor Alignment

You’re probably paying more at the pump if your steering wheel isn’t sitting straight. Similar to low tire pressure, misaligned wheels add unnecessary resistance to your everyday drive. As a result of this constant resistance, your vehicle’s engine must exert more effort, which reduces fuel economy.

A close-up picture of a hand slamming on a car horn.

12. Poor Driving Habits 

Why are you getting bad gas mileage? Well, it could be your driving. Especially in busy cities or during rush hour, many drivers like to accelerate through a greenlight as fast as possible. And while getting stuck at a red is never fun, sudden acceleration uses more gas and wears out your vehicle more. 

According to the U.S. Department of Energy, aggressive driving reduces gas mileage by 30% on highways and 40% on city streets. Remember, driving too close to the vehicle in front of you doesn’t just reduce your fuel economy, it also increases the chance of an accident and fuel leaks in your vehicle.

Tip: since the risk of sudden braking increases if you drive too close to the vehicle in front of you (i.e. tailgating), the general recommendation is to keep three seconds’ worth of space between your vehicle and the vehicle in front of you.


Curious whether blasting your AC on a hot day or leaving your vehicle in park to warm it up on a cold morning is causing you to fill up on gas more frequently? We’ve compiled a few popular MPG-related questions below.

How Much Gas Does Idling Use?

Idling can use more gas than many drivers think. Depending on the type and size of the engine, letting your vehicle idle can use up to ½ gallon of fuel per hour. That doesn’t seem like much, and maybe you’re thinking to yourself that allowing your vehicle to idle for an hour seems excessive anyway, but simply allowing it to idle for a few minutes every day can quickly build up.

Myth: some still think an engine needs to idle before driving, especially in cold weather. However, unless you’re driving an older vehicle with a carburetor, which arguably needs time to warm up to get the right mix of air and fuel in the engine, a fuel-injected engine has sensors to ensure it gets the perfect mixture every time, no matter how cold it is outside.

Does AC Waste Gas?

Similar to many other features of your vehicle, your car’s air conditioning system does add a slight load to the engine, but not enough to matter. According to the American Automobile Association (AAA), using your AC on a hot day has such a minimal effect on your vehicle’s MPG that it’s essentially unrecognizable.

Does Changing Spark Plugs Improve Gas Mileage?

Changing your vehicle’s spark plugs can have a noticeable effect on your vehicle’s fuel economy. Misfiring spark plugs can reduce fuel efficiency by as much as 30%. As a general recommendation, spark plugs should be replaced about every 30,000 to 90,000 miles.

Can New Motor Oil Increase MPG?

Having your vehicle’s oil changed on a regular basis can increase your MPG. Since oil serves as a lubrication for the metal components of your vehicle’s engine, consistent oil changes reduce friction and improve engine efficiency, which reduces the amount of labor and gas consumed by the engine.

What Is A Good Mileage Per Gallon?

While some electric vehicles are boasting around 60 MPG, a normal petrol engine that gets at least 30 MPG is still considered good.

At K&W, our tire and auto services experts are ready to help you get the most out of your vehicle. Contact us today and find the K&W store in your area!