man filling up car at gas pump when gas mileage gets bad

8 Reasons Your Gas Mileage Is Bad


Paying for gas at the pump can be painful enough. But when the cost of every fill-up increases (even though the price is the same), the impact can feel excruciating—especially on your budget. If you’ve been asking, “why is my car getting terrible gas mileage all of a sudden?” it’s time to check your car for one of these problems (or to contact a mechanic to diagnose more serious gas mileage issues).

Low tire pressure

Low tire pressure is one of the most common causes of low gas mileage. To get the most mileage from every tank of gas, check the owner’s manual or decal on the inside of the driver door for the manufacturer’s recommended tire pressure (not the PSI printed on the tire!). Check the tire pressure often (directions for checking tire pressure here) and add air as needed. If you can’t or don’t have time to check the tire pressure, ask your mechanic to check the tire pressure at your next appointment.

Oxygen sensor issues

The proper fuel-air mixture is essential for a smooth-running engine. It’s also important element of getting good gas mileage. Oxygen sensors, which are in vehicles produced after the 80’s, produce data that is relayed to the vehicle computer. The data plays a key role in determining the amount of fuel is added. When an oxygen sensor fails, the engine may burn rich and cause a drop in fuel mileage. If you notice decreased fuel mileage or suspect faulty oxygen sensors, schedule an appointment with a mechanic.

Clogged air filters

A car’s air filter ensures proper air flow to the engine. When the air filter is dirty, the engine compensates by adding more fuel and burning rich. Besides decreased gas mileage, a misfiring engine, lit check engine light, or a strong smell of gas are signs of a clogged air filter. On average, air filters should be replaced every 3 years or 30,000 miles. If you drive your car on dusty roads, plan on scheduling an appointment to replace the air filter more often.

Old spark plugs

Because spark plugs are responsible for the spark that ignites the fuel-air mixture in your engine, worn spark plugs can cause your fuel mileage to drop. Spark plugs should be replaced every 30-100,000 miles; the exact mileage depends on the type of spark plugs. As spark plugs age, the gap at the top of the spark plugs widens. The wider gap can also cause slow starts, engine surges and misfires, and slow acceleration.  

Faulty fuel injectors

Fuel injectors are responsible for the exact process the name implies. Because these car parts are responsible for adding fuel, faulty or clogged fuel injectors play a part in fuel mileage. In addition to a decrease in fuel mileage, you should schedule an appointment to find out if fuel injectors are the problem if the engine misfires or accelerates slowly. A check engine light is also a sign that it’s time to schedule an appointment with an experienced mechanic.

Fast accelerations & sudden braking

Not every cause of bad gas mileage is related to faulty car parts. Stomping on the gas pedal and sudden braking can send you to the pump more often. Unless absolutely necessary, try to avoid sudden accelerations and braking on the road. Smooth driving and accelerations are a key part of maintaining good gas mileage.

Excessive idling

Leaving your car running once in a while won’t cause gas mileage to drop, but excessive amounts of engine idling can negatively impact gas mileage. Even just a few minutes of idling can burn as much gas driving a mile down the road.

Hot weather

The hot weather itself doesn’t affect gas mileage, but using the air conditioning all the time does. Excessive AC use can decrease gas mileage. Instead of turning on the air conditioner, try to open windows as much as possible—except for when traveling at high speeds. Open windows at high speeds can impact the aerodynamics of the car and decrease gas mileage.

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