18 Factors That Affect Your Car’s Fuel Economy

Photo of author
Written By Thomas Anderson

As vehicles rack up more miles, it’s natural to wonder why your car’s gas mileage seems to be declining. After surpassing the initial 100,000-mile mark, a car that once boasted an EPA-rated 28 MPG might now struggle to achieve 20 MPG—or even less! This decrease in fuel efficiency means more frequent visits to the gas pump and heavier burdens on your wallet. If you’ve observed a decline in gas mileage as your car ages, there are numerous things you can do to conserve fuel and enhance overall fuel efficiency. We explore 18 factors that impact your vehicle’s fuel economy.

Frequent Cold Weather Travel

In chilly weather conditions, your vehicle’s engine requires even more time to reach its optimal operating temperature, reducing fuel efficiency. It’s important to note that idling your car to warm it up doesn’t enhance gas mileage; instead, it wastes fuel unnecessarily. As the Pew Research Center indicates, a slim majority of Americans (54%) who prefer colder climates express a desire to reside in Denver, while nearly half (45%) consider Seattle an attractive destination. Forty percent opt for Portland, Oregon.


Everyone knows that getting a speeding ticket can hit your wallet, but speeding itself can also burn through gas faster. When you drive at speeds of around 80 MPH or above, you’re causing more air resistance, which lowers your fuel efficiency. So, you are not only risking a fine but also wasting fuel unnecessarily by driving at high speeds. It’s worth slowing down to save both money and gas.

Excessive Short Trips

Optimal engine efficiency is achieved after a thorough warm-up process. Short-distance commutes limit this warming-up period, leading to increased overall fuel consumption. According to Statista Consumer Insights, 73 percent of American commuters rely on their personal vehicles for daily travel between home and work, marking it the prevailing transportation choice. Comparatively, 13 percent of the 7,649 surveyed individuals opt for public transportation, with 11 percent favoring biking. While alternatives have gained traction since 2019, none pose a substantial challenge to the dominance of cars in the American commute. For those living proximate to their workplace, embracing biking offers manifold benefits for the body, mind, and spirit.

Underinflated Tires

Reduced tire pressure is a prevalent factor contributing to declining MPG since an underinflated tire encounters heightened rolling resistance against the road and a slightly diminished effective diameter. While modern vehicles incorporate a tire pressure monitoring system (TPMS) to alert drivers of low tire pressure, the warning typically activates after a substantial loss of PSI. Even a mere five PSI below the recommended pressure can induce drag and decrease fuel efficiency, even before the TPMS alerts. Maintaining optimal tire pressure by routinely checking and inflating tires as necessary remains one of the simplest strategies to uphold fuel efficiency.

A Dirty Car

Ensuring your vehicle’s exterior stays clean is vital for optimal fuel efficiency. Research indicates that the grime buildup on your car’s surface can significantly impede aerodynamics, resulting in heightened drag and decreased fuel economy. In fact, studies suggest that a dirty exterior could diminish your fuel efficiency by up to 10%, debunking the misconception that dirt enhances aerodynamic performance. Therefore, it’s essential to regularly wash your car to mitigate these adverse effects and preserve fuel efficiency.

Heavy Braking or Acceleration

Engaging in aggressive driving habits causes your vehicle to shift gears more rapidly than ideal for fuel efficiency. Based on EPA testing, frequent heavy braking and rapid acceleration can significantly reduce fuel economy, potentially by as much as 33% during highway travel. Maintaining a steady and moderate pace while driving is important to maximize fuel efficiency and minimize unnecessary fuel consumption.

Worn Out Piston Rings

Piston rings inside your engine cylinders play a crucial role by sealing against the cylinder walls, enabling compression. Over time, worn-out piston rings fail to maintain this seal, causing a loss of engine pressure and reduced fuel efficiency. Engine oil not only lubricates the piston rings but also aids in maintaining optimal fuel efficiency. Ensuring regular oil changes with the manufacturer-recommended oil type, as specified in your owner’s manual, is the best course of action to address this issue.

Clogged or Damaged Fuel Injectors

One of the primary reasons for a decline in fuel efficiency often lies in dirty fuel injectors. Fuel injectors, responsible for spraying fuel into each engine cylinder, require a precise spray pattern to effectively mix with air and ignite within the engine. When fuel injectors get dirty or clogged, their spray becomes inefficient, akin to a poorly functioning shower head. This inefficiency can rapidly diminish engine performance and fuel economy. Fortunately, cleaning the fuel injector nozzles can often resolve the issue. However, replacement may be necessary if internal damage is the root cause of the problem.

High Vehicle Weight or Towing Weight

When your vehicle carries a heavy load, it places additional strain on the engine, requiring it to exert more effort. This added weight might stem from various sources, such as luggage or bike racks, a substantial interior payload, or trailer towing. It’s essential to be mindful of the extra burden on your engine when carrying such loads, as it can impact fuel efficiency and overall vehicle performance. Taking steps to minimize unnecessary weight can help maintain optimal driving conditions. In an NBER paper, Pounds that Kill: The External Costs of Vehicle Weight, it is found that getting hit by a car that’s 1,000 pounds heavier leads to a 47% higher chance of fatal injury.

Dirty Mass Airflow Sensor

Mass airflow sensors play a critical role in determining the volume of air entering the engine, akin to oxygen sensors. This data is then relayed to the vehicle’s computer, which adjusts fuel injection to maintain the correct air-to-fuel ratio. However, if the airflow sensor becomes contaminated, the computer may misinterpret data, resulting in an improper air-fuel mixture and reduced fuel efficiency or engine stalling. It’s advisable to periodically clean mass airflow sensors using a specialized cleaning spray to ensure optimal engine performance.

Poor Alignment

Continuously driving with a crooked steering wheel may indicate that your wheel alignment affects your wallet at the gas station. Unlike low-pressure tires, misaligned wheels generate resistance, hindering straight road travel. This constant battle against resistance demands extra effort from the engine, ultimately diminishing fuel efficiency and draining your wallet. To prevent these unnecessary expenses, it’s wise to promptly address any steering wheel misalignment, ensuring smoother travels and optimal fuel economy.

Dirty Oxygen Sensor

While older vehicles rely on carburetors to regulate the air-to-fuel ratio for combustion, newer models since approximately 1996 utilize oxygen sensors instead. These sensors gauge the richness or leanness of exhaust gases exiting the engine and relay this information to the car’s computer for fuel adjustment. According to Edmunds, a contaminated oxygen sensor can yield inaccurate readings, leading to excessive fuel consumption and up to a 40% decrease in efficiency. Malfunctioning oxygen sensors often trigger the check engine light and should be inspected and potentially replaced before reaching 100,000 miles. Fortunately, replacing oxygen sensors is relatively inexpensive, aiding in fuel savings and emissions control.

Engine Size

Engine size plays a significant role in determining fuel economy, with larger engines consuming more fuel than smaller ones. Opting for a smaller engine size can help maximize fuel efficiency, as operating requires less fuel. Prioritizing a smaller engine size ensures you get the most out of your fuel, ultimately leading to cost savings and reduced environmental impact. Therefore, when selecting a vehicle, consider engine size as a crucial factor in achieving optimal fuel economy.

Worn or Stuck Brakes

Driving with deteriorated brake components poses safety risks and potential harm to your MPG. Sticky brakes, such as a stuck caliper or adhesive brake pads, introduce resistance against your vehicle’s forward movement. This brake drag forces the engine to contend continuously with the brakes, significantly reducing fuel efficiency. To mitigate these issues, it’s imperative to conduct regular brake checks or schedule a thorough brake inspection at Firestone for peace of mind and optimal vehicle performance.

Old or Incorrect Engine Oil

The belief that older cars require thicker engine oil to prevent leaks is a common misconception. While “high mileage engine oils” contain additives to condition seals and slightly increase viscosity, thicker oil creates more resistance between engine parts, reducing fuel efficiency. Proper maintenance, including regular oil changes with the manufacturer-recommended oil type, is crucial for modern engines. Although high mileage oils may help with oil leaks and consumption, they may sacrifice some potential fuel efficiency gains. Prioritizing correct oil selection and maintenance routines is essential for optimal engine performance.

Clogged Fuel Filter

Fuel filters are critical in safeguarding the engine by trapping harmful contaminants present in the fuel, preventing them from circulating and causing damage to vital components like fuel injectors. A congested fuel filter diminishes fuel pressure, leading to subpar engine performance. Particularly for older vehicles, it’s crucial to adhere to a regular maintenance schedule and replace fuel filters roughly every two years or every 30,000 miles to ensure optimal engine health and longevity.

Old Air Filter

Engines rely on air intake to function effectively. A heavily soiled or blocked engine air filter impedes this airflow, suffocating the engine’s ability to “breathe.” In response, older engines consume more fuel to maintain speed parity. In comparison, newer engines may struggle to compensate, leading to diminished performance. This issue is prevalent in older vehicles equipped with carburetors. It’s recommended to replace engine air filters every 15,000 to 30,000 miles, but consult your owner’s manual for precise intervals or assess it during your next oil change appointment.


Fuel economy is significantly influenced by the level of aerodynamic drag experienced by a vehicle. Cars with sleek, aerodynamic profiles generally boast better fuel efficiency than boxy vehicles with sharp edges. To maximize gas savings, consider investing in a car with superior aerodynamics. This choice contributes to reduced fuel consumption and promotes a more environmentally friendly driving experience. Prioritizing aerodynamic design when selecting a vehicle can lead to long-term cost savings and enhanced fuel efficiency.