Why Would Gas Mileage Decrease on My Car? (2023)

If you've noticed a decrease in your gas mileage and are wondering why, you've come to the right place. We have thoroughly researched and compiled a list of the top reasons why your car might be getting worse gas mileage than you're used to. By identifying these factors, you can easily pinpoint the problem and take the necessary steps to fix it.

1. Deferred Maintenance

One of the most common reasons for a decrease in gas mileage over time is deferred maintenance. When you neglect routine maintenance tasks, your car's engine has to work harder, resulting in increased fuel consumption. Here are some specific areas of maintenance that can impact gas mileage:

Engine Air Filter: Over time, your car's engine air filter can become clogged with dirt and debris, restricting the airflow into the engine. This forces the engine to work harder and decreases gas mileage [[SOURCE 1]].

Spark Plugs & Wires: Fouled or damaged spark plugs can lead to a decrease in gas mileage. Regularly replacing spark plugs as part of a standard tune-up can help maintain optimal fuel efficiency [[SOURCE 1]].

Oxygen Sensor: A faulty oxygen sensor can cause your car's computer to inject more fuel than necessary, resulting in decreased gas mileage. Replacing a malfunctioning oxygen sensor can help improve fuel efficiency [[SOURCE 1]].

Fuel Injectors: Clogged or damaged fuel injectors can disrupt the proper fuel-air mixture in the engine, leading to decreased gas mileage. Cleaning or replacing fuel injectors can restore optimal fuel efficiency [[SOURCE 1]].

Fuel Filter Failure: A clogged or damaged fuel filter can impede the flow of clean fuel to the engine, reducing gas mileage. Regularly replacing the fuel filter can help maintain fuel efficiency [[SOURCE 1]].

Bad Piston Rings: Worn or damaged piston rings can cause lower compression in the engine, resulting in decreased power and fuel efficiency. Repairing or replacing faulty piston rings can improve gas mileage [[SOURCE 1]].

Brakes Dragging: If your brakes are dragging, it creates resistance that makes your car work harder and decreases gas mileage. Regular brake maintenance and addressing dragging issues promptly can help maintain fuel efficiency [[SOURCE 1]].

Using the Wrong Engine Oil: Using the wrong engine oil can lead to various problems, including decreased gas mileage. Consult your vehicle's user manual to ensure you're using the recommended oil for optimal performance and fuel efficiency [[SOURCE 1]].

Bad Fuel Lines or Fittings: Leaking fuel due to damaged fuel lines or fittings not only decreases gas mileage but also poses a safety risk. Promptly addressing any fuel leaks is crucial for both fuel efficiency and safety [[SOURCE 1]].

2. Tire and Wheel Issues

The condition of your car's tires and wheels can significantly impact gas mileage. Here are some tire and wheel-related factors to consider:

Proper Tire Inflation: Ensuring that your tires are properly inflated can reduce rolling resistance, making it easier for your car to move and improving gas mileage. Regularly check and maintain the recommended tire pressure for optimal fuel efficiency [[SOURCE 1]].

Tire Tread Wear: Worn-out tires with reduced tread depth can negatively affect gas mileage. As tires age, they become less efficient and can contribute to decreased fuel efficiency. Consider replacing tires that no longer have sufficient tread depth [[SOURCE 1]].

Wheel Alignment: Misaligned wheels can cause increased rolling resistance, leading to decreased gas mileage. Regular wheel alignments can help ensure optimal tire contact with the road surface, improving fuel efficiency [[SOURCE 1]].

3. Driving Habits

Your driving habits can have a significant impact on gas mileage. By adopting certain driving practices, you can improve fuel efficiency. Here are some tips:

Smooth Acceleration and Braking: Avoid rapid acceleration and hard braking, as these actions consume more fuel. Gradually accelerating and decelerating can help improve gas mileage [[SOURCE 1]].

Maintain a Steady Speed: Driving at a consistent speed, particularly within the optimal range of 50mph to 55mph, can maximize fuel efficiency. Avoid unnecessary speed fluctuations to achieve better gas mileage [[SOURCE 1]].

Avoid Stop-and-Go Traffic: Frequent stop-and-go traffic can decrease gas mileage due to the constant acceleration and deceleration. Whenever possible, choose routes with less congestion or consider carpooling to reduce the impact of stop-and-go traffic on fuel efficiency [[SOURCE 1]].

4. Fuel Quality

Using poor-quality or contaminated fuel can have adverse effects on both your car's engine and gas mileage. Here's what you need to know:

Fresh Fuel: Using fresh, high-quality fuel is essential for optimal engine performance and fuel efficiency. Avoid using old or stale fuel that may contain impurities or water, as it can negatively impact gas mileage and potentially damage the engine [[SOURCE 1]].

Choose Reliable Gas Stations: Selecting reputable gas stations known for their fuel quality can help ensure that you're getting clean and uncontaminated fuel. Stick to a trusted gas station to minimize the risk of fuel-related issues [[SOURCE 1]].

5. Changes in Vehicle Aerodynamics

Modifications or additions to your vehicle can alter its aerodynamics, potentially leading to decreased gas mileage. Consider the following factors:

Lift Kit: Installing a lift kit raises your vehicle's height, which can disrupt the airflow around it. This change in aerodynamics can result in decreased gas mileage [[SOURCE 1]].

Roof Rack: While roof racks are convenient for carrying additional cargo, they create drag that can reduce fuel efficiency. Remove the roof rack when not in use to improve gas mileage [[SOURCE 1]].

Cargo Box: Similar to roof racks, cargo boxes mounted on the roof can increase drag and decrease gas mileage. Use them only when necessary and remove them when not in use [[SOURCE 1]].

Roof Cross Bars: Roof cross bars are essential for mounting roof racks or cargo boxes, but they can also contribute to increased drag. Consider removing them when not needed to improve fuel efficiency [[SOURCE 1]].

Off-Road Bumper: Heavy off-road bumpers not only add weight to the vehicle but can also disrupt aerodynamics. This can lead to decreased gas mileage. Evaluate whether the benefits of an off-road bumper outweigh the impact on fuel efficiency [[SOURCE 1]].

6. Low Compression

If your car's engine has low compression, it can significantly impact gas mileage. Compression is crucial for generating power in the engine. When compression is low, the engine works harder and consumes more fuel. Addressing low compression issues may require professional assistance to restore optimal fuel efficiency [[SOURCE 1]].


In conclusion, there are several factors that can contribute to a decrease in gas mileage. By addressing deferred maintenance, ensuring proper tire and wheel conditions, adopting fuel-efficient driving habits, using high-quality fuel, considering vehicle aerodynamics, and addressing engine issues, you can improve your car's gas mileage. If you're experiencing persistent issues or need further assistance, it's recommended to consult a professional mechanic for a thorough diagnosis and appropriate solutions. Take proactive steps to maintain your car's fuel efficiency and enjoy the benefits of improved gas mileage.

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