Why Your Car Is Using More Gas Than It Used To

Common reasons your fuel economy has gotten worse

A car that suddenly drinks more gas than before is usually telling you something specific is wrong. The most common culprits are a clogged air filter, worn spark plugs, low tire pressure, a faulty oxygen sensor, or an issue with your fuel injectors. Each one reduces how efficiently your engine burns fuel, forcing it to work harder to move the same distance.

Less obvious but equally common: you may have switched to a heavier grade of motor oil than your car needs, or your brakes may be dragging slightly because the brake pads are worn or the caliper is sticking. A misaligned wheel also increases rolling resistance. Any of these will show up at the pump before you notice it while driving.

Key Takeaways

• Low tire pressure is the single easiest thing to check yourself and often accounts for a noticeable drop in fuel economy.
• A clogged air filter, worn spark plugs, or a faulty oxygen sensor will make your engine work harder and burn more fuel to go the same distance.
• Dragging brakes, misaligned wheels, and using the wrong motor oil all increase resistance and reduce how far each gallon takes you.
• A mechanic can run a diagnostic scan to pinpoint sensor problems and other issues you cannot see yourself.

Check your tire pressure first

Underinflated tires are the most common reason for sudden fuel economy loss, and it is the easiest thing to fix yourself. When a tire is even 5 pounds per square inch (psi) below the recommended pressure, your car has to push harder against the road, burning more fuel to maintain speed. A tire that is 10 psi low can reduce your fuel economy by 3 to 5 percent.

Find the correct pressure for your car on a sticker inside the driver's door jamb or in your owner's manual—not on the tire itself, which shows the maximum pressure. Check your tire pressure when the tires are cold, before you have driven the car or at least three hours after driving. Use a tire gauge from any gas station or auto parts store. If any tire is low, inflate it to the recommended pressure and check again in a few days to see if it is losing air, which could mean a slow leak.

Air filter, spark plugs, and fuel injectors

A clogged air filter forces your engine to work with less oxygen, making it burn more fuel to produce the same power. Most air filters should be replaced every 15,000 to 30,000 miles, depending on driving conditions and how dusty your area is. If you drive on unpaved roads or in very dusty conditions, check yours more often. A new air filter costs $15 to $40 and takes a mechanic minutes to install.

Worn spark plugs misfire or fire inconsistently, which means some fuel does not burn completely. This wastes gas and often triggers a check engine light. Spark plugs typically last 30,000 to 100,000 miles depending on the type. Fuel injectors that are clogged or leaking spray fuel unevenly into the cylinders, again wasting fuel. A fuel system cleaning can sometimes restore injector function, but if an injector is truly faulty, it will need to be replaced.

Oxygen sensor and engine diagnostics

Your oxygen sensor tells your engine how much fuel to inject based on how much oxygen is in the exhaust. A faulty sensor causes the engine to run too rich (too much fuel) or too lean (too little fuel, which then causes it to compensate by burning more). A bad oxygen sensor almost always triggers a check engine light, which a mechanic can read with a diagnostic scanner.

If you see a check engine light or your fuel economy has dropped more than 10 percent, a diagnostic scan is worth the cost—usually $75 to $150. The scan will read the fault codes stored in your engine computer and tell you exactly what is wrong. Many auto parts stores offer free or low-cost scans, though the results are sometimes less detailed than what a mechanic's equipment will show.

Brakes, alignment, and motor oil

Dragging brakes create constant friction that your engine has to overcome, burning extra fuel even when you are not pressing the pedal. Worn brake pads, a stuck caliper, or a malfunctioning brake proportioning valve can all cause this. If your car pulls to one side when you brake, or if you smell burning when braking, have your brakes inspected. Brake work is not optional—it affects your safety.

Misaligned wheels increase rolling resistance the same way underinflated tires do. If your car pulls to one side while driving straight, or if your tires are wearing unevenly, get an alignment check. Using a heavier motor oil than your owner's manual specifies also increases engine friction. If you recently had an oil change and your fuel economy dropped, check what grade was used and compare it to what your manual calls for.

Driving habits that look like a fuel economy problem

Sometimes the car is fine but your driving has changed. Frequent short trips, heavy traffic with lots of braking and accelerating, or driving at highway speeds above 55 mph all burn more fuel than steady, moderate driving. Cold weather also reduces fuel economy because your engine needs more time to warm up and oil is thicker when cold. If your fuel economy dropped only in winter, this is normal and will improve when the weather warms.

Carrying extra weight—a roof rack, a full trunk, or towing a trailer—also increases fuel consumption. The heavier your car, the more fuel it needs to accelerate and maintain speed. If you have added cargo or equipment recently, removing it will restore some of your lost fuel economy.

When to see a mechanic

If your tire pressure is correct and your driving habits have not changed, schedule an inspection. Bring a record of your fuel economy over the past few fill-ups if you have it—a sudden drop is more telling than a gradual one. A mechanic can visually inspect your air filter, check your brakes and alignment, and run a diagnostic scan to catch sensor problems. The cost of finding the problem early is usually far less than the cost of fuel wasted by driving with a faulty part.

If your check engine light is on, do not ignore it. The light means your engine computer has detected a problem that is likely affecting your fuel economy or emissions. Have the code read as soon as you can.

Frequently Asked Questions

How much does low tire pressure actually cost me in gas?

A tire that is 10 psi below the recommended pressure typically reduces your fuel economy by 3 to 5 percent. On a car that normally gets 25 miles per gallon, that means you lose about 0.75 to 1.25 miles per gallon. Over a year of driving, that adds up to dozens of dollars in wasted fuel.

Can I fix a clogged fuel injector myself?

You can buy fuel system cleaner additives to pour into your gas tank, which may help if the injectors are only slightly clogged. However, if an injector is severely clogged or leaking, a mechanic will need to remove and clean or replace it. A professional fuel system cleaning costs $50 to $200 depending on your car.

What does a check engine light have to do with gas mileage?

The check engine light indicates your engine computer has detected a fault—often in the oxygen sensor, catalytic converter, or emissions system. These faults almost always cause your engine to burn fuel inefficiently. Having the code read will tell you what is wrong and whether it is affecting your fuel economy.

Is it normal for fuel economy to drop in winter?

Yes. Cold engines need more fuel to start and warm up, and cold oil is thicker and creates more friction. Winter fuel blends also have different properties. Expect 10 to 15 percent worse fuel economy in cold weather. This improves as temperatures rise.

How often should I replace my air filter?

Most manufacturers recommend every 15,000 to 30,000 miles, but this depends on your driving conditions. If you drive on dusty roads or in a dry climate, check yours every 10,000 miles. A mechanic can inspect it during routine service and tell you if it needs replacing sooner.