Car Engine

Why Is My Car Getting Bad Gas Mileage All Of A Sudden

Mike Cross
Updated Aug 22, 2021
Why Is My Car Getting Bad Gas Mileage All Of A Sudden

Gas mileage is very important when it comes to your car. That is because you want your car to be as fuel efficient as possible to help save you money.

For those that aren’t aware, gas mileage is the number of miles you can go per gallon of gas in your car. It is sometimes written as MPG which means Miles per Gallon and a vehicle's gas mileage normally stays pretty consistent.

That is why if your gas mileage changes suddenly you should be aware of it and investigate. In this article, we will look at possible reasons why your gas mileage might change suddenly and things you can do to fix it and improve your gas mileage


Why Is My Car Getting Bad Gas Mileage All Of A Sudden
Why Is My Car Getting Bad Gas Mileage All Of A Sudden


There are nine main reasons why your cars gas mileage might drop suddenly or become bad. These issues are

  • Issues with Tires
  • Low Oil Level
  • Dirty Oxygen Sensor
  • Defective Mass Airflow Sensor
  • Faulty Fuel Pump
  • Clogged Fuel Injectors or Air Filter
  • Spark Plug Issues
  • Stuck Caliper or Shoe
  • Bad Driving

We will go into detail on each one of these issues below, so keep reading, or if you think you know what might be causing your fuel to lose issues, then you can jump straight to that and read about it to see if you were right or not.

Issues with Tires

The first thing that can lead to a sudden loss of fuel efficiency that we will look at is issues with your tires. The main thing with tires is that they need to be under the proper pressure. This is because if they are low on pressure, then your car is harder to move to cause the engine to work harder.

The car is harder to move when tire pressure is low because the wheels won’t be round. Tire pressure is an easy problem to check for and solve though. All you need is a cheap little tire pressure gauge, and you can measure the pressure. If the pressure reads low than the amount stamped onto the driver side door, then you need to add air which is easy to do.

If it isn’t low pressure though, then it still might be an issue with your tires. If the tires are not aligned right or off balance, then this can also make your car harder to move.

The way you can check for misaligned tires without taking the car in is let go of the wheel when driving. The car should stay straight if it doesn’t mean you have an alignment or balance issue.

One last way to check is to look at the tires and see if the tread is wearing evenly. If it is, then you are fine, but if parts of the tire look more wore than others, then you will need to have your tires rotated and aligned.

Low Oil Levels

The next thing that could lead to worse gas mileage is low or old oil. This is because the oil helps lubricate the engine and if you don’t have enough or it is old and not doing its job the engine will have to work harder, and it is getting wear and tear. Any time the engine has to work harder, it will burn more gas.

To prevent this make sure to change your oil regularly and check it regularly too. Most all vehicles have a dipstick under the hood, and all you do is pull it out and look at it to make sure the oil is at the right spot. It is a good idea to do this twice and wipe it off with a towel between each time to make sure you are getting an accurate reading.

Dirty Oxygen Sensor

Dirty sensors can also lead to the car burning more fuel suddenly. The oxygen sensor is responsible for measuring the air in the cylinder of the engine before combustion and then adding in the right amount of gas.

If the sensor is reading wrong, then too much gas may get added. This leads to a loss of fuel efficiency. That is why it is a good idea to check your sensors and clean those if you start noticing mileage lose and you don’t have the other mentioned problems.

Defective Mass Airflow Sensor

Another sensor to check beside the oxygen sensor that could be bad is the mass airflow sensor. It could be dirty as well or not working right. A lot of times if a sensor stops working right you will get a check engine light, but not always.

That is why it is key to check all sensors if your car starts burning more gas than normal. Your car has a variety of sensors, and they all control how much gas goes into the fuel mixture. Outside of gas mileage though it is important to get your sensors checked because damage can happen to your engine if it burns the wrong fuel mixture for too long.

Faulty Fuel Pump

The next thing after the sensors you should check is your fuel pump. Your fuel pump might be pumping too much gas into the engine. This can come about for a variety of reasons including the pump not pumping fuel or oil right.

A bad fuel pump can be diagnosed because your car is likely to stall or have backfired if it is going really bad. At first, it may be hard to notice, but if you have noticed bad gas mileage and checked other things with no improvement, then you should check your fuel pump to make sure it is working right.

Clogged Fuel Injectors or Air Filter

Like most of the other problems, this problem goes back to having a too rich fuel mixture as the problem for your car burning more gas than normal. A dirty air filter can lead to false readings of air just like a bad sensor.

A bad fuel injector can also do the same thing because it might fire twice if the sensor doesn’t read enough gas in the cylinder. A dirty fuel injector also might stay open leading to more gas coming out.

It is key to replace your air filter when it gets dirty and also to clean your fuel injectors on occasion to make sure they stay working right. Another way to help your fuel injectors last longer is to put better gas in your car that burns cleaner.

Spark Plug Issues

Like with the fuel injectors, if you have dirty or bad spark plugs, then the explosion in the engine may not be accurate leaving extra gas in the engine. This isn’t good because it leads to engine damage and also the loss of fuel efficiency.

If you notice fuel lose, then checking your spark plugs is a good idea. Most last for a long time 100,000 plus miles, but eventually, they do need to be changed. When checking to make sure the gap is right and that they aren’t burnt up or corroded. If one is bad, then it is a good idea to change them all to keep the car running smoothly.

Stuck Caliper or Shoe

The next issue goes back to the wheels and away from the engine and that is you might have a stuck caliper or shoe when we say that we are talking about your breaks. You might have a break that still has the pad on the rotor. This will make it harder to drive and can be dangerous because it can make controlling the car harder.

When checking the tire pressure look at the brakes and see if you notice any issues. If it looks like a caliper or shoe is in the wrong position, then take the tire off and check it out. Make sure you also have the proper amount of brake fluid in your car because this helps lubricate the breaks and keeps them from getting stuck.

Bad Driving

The last issue is bad or inconstant driving. Your speed effects fuel efficiency a lot, so if you sped recently, then that might be why you had the fuel lose. Also, accelerating uses the most gas, so if you have been slowing down and speeding up more than normal, then that could be where your gas went as well. Lastly, if you have been idealing more than normal, then that might be the reason you have lost fuel efficiency. That is because cars burn gas when just sitting too.

Since you’re driving can affect fuel so much it is okay to have varying miles per gallon on occasion, but major drops should be checked. Especially if you notice a repeated change in fuel efficiency.


Luckily for you, now you know the things to check if you notice a fuel drop off. You know that while your driving style affects it, it also might be a bigger issue. You now know to check your tires for proper pressure and alignment, and check your breaks. You also know to clean and check your sensors, air filter, spark plugs, and fuel injectors. Lastly, if now of these are the issues, you might have a faulty fuel pump after reading this though you are more aware of why your car might be getting bad gas mileage all of a sudden. More importantly, though you also know how you can fix the issues and different things you can do about it.

Mike Cross
Life is too short to drive with stock audio

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