Have you noticed a decrease in your gas mileage, and are on the hunt for why? You have come to the right place then. We have worked hard to try to find all of the different reasons why your car might be getting worse gas mileage than you are used to, so that you can easily pinpoint the problem, and get it fixed! Some are as simple as removing an aftermarket part from your vehicle, others are a little more difficult, and can even require an engine replacement.

So without more to do, here are the top reasons why your gas mileage is getting worse by the day!

1. Deferred maintenance

This is one of the most common reasons for gas mileage to decrease over time. When you don’t keep up with routine maintenance, your car’s engine has to work harder, which uses more gas.

So many people out there don’t take care of their vehicles, because they believe it to be cheaper than just taking care of their car. The opposite is true though, not only due to a drop in gas mileage, but many of these things can cause catastrophic damage to your vehicle if not taken care of.

  • Engine Air Filter: Over time, your car’s engine air filter will get clogged with dirt and debris. When this happens, it restricts the amount of air that can flow into the engine. Engines use a ton of air to stay running, and are a big part of combustion! Without the air that they need, it has to work harder and decreases their gas mileage.
  • Spark Plugs & Wires: Spark plugs are what ignite the air/fuel mixture in your engine. Over time, they can become fouled or damaged, which can cause a decrease in gas mileage. This is a normal thing to have to fix and should be done as a part of a standard tune-up.
  • Oxygen Sensor: The oxygen sensor is what tells your car’s computer how much fuel to inject into the engine, by reading how much air is being added. When it goes bad, your car will run rich, which means it will use more gas than it should.
  • Bad Fuel Injectors: Fuel injectors are what spray the fuel into your engine, mix this with spark and air, and you have a running engine. Over time, these injectors can become clogged or damaged, which can cause a decrease in gas mileage. This will also cause your engine to run rough, and even stall out.
  • Fuel Filter Failure: The fuel filter protects your engine from dirt and debris in the gas. Over time, it can become clogged or damaged, which can cause a decrease in gas mileage. No matter how good of fuel you buy, it still is going to have some contaminates in it, and a fuel filter does its job, only to a point.
  • Bad Piston Rings: Piston rings seal the combustion chamber in your engine. When they fail, it can cause lower pressure, and combustion in the cylinders, meaning less power can generate. The engine then has to run faster, using more fuel along with it.
  • Your brakes are dragging: This one is pretty simple if your brakes are dragging, it’s going to take more gas to stay up to speed, as your vehicle is actively working against itself. This problem usually presents itself as a decrease in gas mileage, as well as a decrease in performance. You would be surprised how often this happens. Calipers can easily go bad due to road salt from harsh winters, so pay close attention to this if you live in snowy regions.
  • Using the wrong engine oil: If you’re using the wrong oil in your engine, it can cause all sorts of problems. Not only will it decrease gas mileage, but it can cause your engine to run hotter, wear out faster, and even fail. Checking your user manual of your vehicle is the best bet for making sure you have the right oil.
  • Bad fuel lines or fittings (leaking fuel): Leaking gas is going to decrease your gas mileage (or more just lower your fuel amount). Not only that, but it’s incredibly dangerous and should be fixed as soon as possible.

Many of the items under this deferred maintenance can be resolved by a tune-up. You can either do this at home yourself if you are good with vehicles, or you might have to take it to a shop!

2. Tire and Wheel issues

Your car’s tires play a big role in gas mileage. When they are properly inflated, they offer less rolling resistance, meaning your car doesn’t have to work as hard to move. This also helps with traction, so you’re not slipping and sliding all over the place, which also uses more gas.

You may also just need new tires! If your tires are older, and no longer have the tread they used to, you might be losing gas to this issue. As tires age, they get dry rotted, harder and less efficient, easily eating away at your fuel.

If your car is out of alignment, it can cause a decrease in gas mileage as well. This is because your car will be fighting against itself, and the tires will be scrubbing the ground as they roll. This causes more resistance and therefore uses more gas.

3. You need to drive better

This is a gas mileage decrease that you can fix without having to spend any money. If you’re the type of person who speeds everywhere, constantly accelerates and brakes hard, or just drives erratically, your gas mileage will suffer.

But, if you take it easy on the gas pedal and brakes, and drive the speed limit, you’ll see a big improvement in gas mileage.

Generally speaking, if you are hoping to save on gas, you should think about driving in the 50mph to 55mph range, as this is typically where you are going to find the best miles to the gallon! If you are driving a vehicle from the 90s or so, you likely have noticed the 55 on your speedometer is larger than the others, this is because it was recommend to go this speed for maximum gas mileage.

4. You have bad gas

If you’re using gas that is old or has water in it, it can cause all sorts of problems for your car. Not only will it decrease gas mileage, but it can damage your car’s engine. Be sure to use fresh gas, and if you suspect that there might be water in your gas, have it drained and replaced.

You would be surprised to find how many lower end gas stations have these issues! That is why I like to find my favorite gas station, and stick to it.

5. The aerodynamics of your car have changed

This is one aspect many people don’t think about! There are quite a few things that you add to your vehicle that can change the aerodynamics, which is a big part of why vehicles get better gas mileage. Think about these items that you might have installed recently, and whether they may have contributed to your drop in gas mileage.

  • Lift kit: A lift kit raises your vehicle, which changes the way air flows around it. This can cause a decrease in gas mileage as your car is no longer as aerodynamic as it was before.
  • Roof rack: A roof rack is great for hauling things around, but it also causes drag on your car. This drag can decrease gas mileage by up to 25%.
  • Cargo Box: A cargo box mounted to the roof of your car can also cause drag and decrease gas mileage.
  • Roof Cross Bars: Roof cross bars are necessary if you want to mount a roof rack or cargo box, but they also add drag.
  • Off-Road Bumper: These bumpers can be quite heavy, which affects gas mileage, but they also tend to remove different aspects of your vehicle that can affect aerodynamics as well.

6. You have low compression

If your engine has gone completely bad, and you have lost compression, this can cause a massive decrease in gas mileage. Compression is what allows your engine to create the power it needs to run. When it’s low, your car will use more gas and run less efficiently.

7. Stop and Go traffic

If you find yourself in stop-and-go traffic often, it can decrease gas mileage. This is because your car is constantly starting and stopping, which uses more gas than if you were to just maintain a steady speed. This is also why gas mileage on vehicles has a rating for city, highway, and combined.

How do you fix bad gas mileage?

In the end, the best way to fix your gas mileage is to address the different items we talked about in this article! It could mean catching up on your deferred maintenance, changing your driving style, taking off that cargo box and so much more. I would read through this article, decide what might apply to you, and make that change.

If you don’t know where to start, you most definitely should reach out to your mechanic and have a quick chat!


So, as you can see, there are a lot of things that can cause gas mileage to decrease over time. If you’re experiencing a decrease in gas mileage, it’s important to have your car checked out by a professional.

They will be able to diagnose the problem and help you get back on the road with better gas mileage in no time! Thanks for reading!