# Why is my car getting bad gas mileage? **Verdict:** Safe to drive - **Most common cause:** Underinflated tires - **Urgency:** Keep driving - **Typical fix:** $100-$650 - **Causes to check:** 5 ## Quick answer Poor gas mileage most often comes from underinflated tires, followed by driving habits like short trips, idling and cold weather. On the mechanical side, a failing oxygen sensor, a thermostat stuck open, worn spark plugs or a dragging brake caliper are the usual culprits. Start by checking tire pressure and tracking mileage over a few full tanks. ## What is happening? Gas mileage is a measure of how much work the engine does for each gallon. It depends on how efficiently the engine burns fuel and on how much resistance the car has to overcome: rolling resistance from the tires, drag from the brakes and wheels, air drag and weight. A drop in mileage means either the engine is wasting fuel or something is making the car work harder to move. It matters because the causes are often cheap to fix and sometimes affect other things. Low tires wear unevenly and handle worse, a dragging brake can overheat, and a misfire can damage the catalytic converter. Mileage also varies naturally with season, traffic and fuel blend, so a small change is not always a fault. Comparing your real mileage over several tanks with the official estimate for your car on fueleconomy.gov helps show whether the drop is worth chasing. ## What causes it? The 5 most likely reasons 1. **Underinflated tires** (most common). Tires lose air slowly over time and faster in cold weather, so most cars drift below the recommended pressure between checks. Soft tires flex more and roll with more resistance, which takes extra fuel at every speed. Wider tires, off-road tread or larger wheels fitted later also lower mileage compared with the factory setup. How to confirm: Check all four tires cold with a gauge and compare them with the pressure on the driver door jamb label, not the number on the tire sidewall. Fix: Inflate tires to the door jamb pressure. 2. **Short trips, idling, heavy loads and cold weather** (common). An engine uses the most fuel while cold, and many short trips mean it rarely warms up. Winter makes this worse, and winter-blend gasoline holds slightly less energy. Long idling, aggressive acceleration, high highway speeds, roof racks and heavy cargo all add up. A change in routine is a common reason mileage drops with no fault. How to confirm: The drop matches winter, a new commute, more city driving, a roof rack or towing, and there is no check engine light or other symptom. Fix: Combine trips, ease off the gas, remove roof racks and extra weight. 3. **Failing oxygen sensor or thermostat stuck open** (common), $150-$450. Oxygen sensors tell the computer how rich the exhaust is so it can trim fuel. A slow, aging sensor can make the engine run richer than needed. A thermostat stuck open keeps the engine below normal temperature, so the computer adds fuel as if it were cold. The temperature gauge may sit low and the heater feel weak. How to confirm: The check engine light may show P0133, P0171 or P0172 for a sensor, or P0128 for the thermostat. A temperature gauge that stays below its usual spot points to the thermostat. Fix: Replace the oxygen sensor or thermostat. 4. **Worn spark plugs or a misfire** (common), $150-$450. Worn plugs make a weaker spark, so some fuel does not burn completely, and a full misfire wastes fuel in that cylinder entirely. Many plugs last 60,000 to 100,000 miles, and mileage often drops gradually as they wear. A rough idle, hesitation or a flashing check engine light suggests a misfire is involved. How to confirm: The plugs are near or past the interval in your owner's manual, the idle feels rough, or a scan shows P0300 or a cylinder misfire code. Fix: Replace the spark plugs, or the coil if one is failing. 5. **Dragging brake caliper or poor alignment** (less common), $100-$650. A brake caliper with a sticking piston or seized slide pins keeps the pad lightly pressed on the rotor, so the car is always braking a little. One wheel gets hot, the car may pull to one side, and you may smell burning. Badly misaligned wheels also scrub the tires and add drag, showing up as uneven tire wear. How to confirm: After a drive, carefully feel near each wheel without touching the rotor. One much hotter wheel, a burning smell or a pull points to a dragging brake. Fix: Repair or replace the caliper, or align the wheels. ## How to narrow it down at home 1. Check all four tire pressures cold with a gauge and set them to the door jamb label. Remove roof racks or cargo you do not need. 2. Track mileage over at least two full tanks: fill up, reset the trip meter, drive normally, then divide miles by gallons at the next fill-up. 3. Have the codes read even if the check engine light is off, and watch whether the temperature gauge reaches its normal spot after 10 to 15 minutes. ## Is it safe to drive? > **Safe to drive.** Yes, poor gas mileage alone is not a safety problem. Get it checked soon if it comes with a check engine light, a fuel smell, a hot wheel, a pull to one side or a temperature gauge that stays low, since those causes can worsen or damage other parts. ## When does it become urgent? - The check engine light is flashing, which signals a severe misfire: reduce speed and get it checked right away. - You smell raw gasoline inside or around the car, which can mean a fuel leak: stop and park away from people. - One wheel is much hotter than the others or you smell burning brakes. - Mileage drops suddenly by a large amount from one tank to the next. - Black smoke comes from the exhaust, a sign the engine is running very rich. ## What will the repair cost? | Cause | Fix | Typical cost | | --- | --- | --- | | Failing oxygen sensor or thermostat stuck open | Replace the oxygen sensor or thermostat | $150-$450 | | Worn spark plugs or a misfire | Replace the spark plugs, or the coil if one is failing | $150-$450 | | Dragging brake caliper or poor alignment | Repair or replace the caliper, or align the wheels | $100-$650 | Detailed breakdowns: [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement), [thermostat replacement cost](/cost/thermostat-replacement), [spark plug replacement cost](/cost/spark-plug-replacement), [brake caliper replacement cost](/cost/brake-caliper-replacement), [wheel alignment cost](/cost/wheel-alignment). ## What to tell your mechanic Tell the shop how much your mileage dropped, over how many tanks, whether it was sudden or gradual, and any changes in driving, season, tires or loads. Mention warning lights, a low temperature gauge, pulling or burning smells. They will read codes and fuel trims, check oxygen sensors and coolant temperature, inspect plugs, and check for dragging brakes and alignment. ## How to prevent it - Check tire pressure monthly and before long trips, using the door jamb label. - Replace spark plugs and the engine air filter at the intervals in your owner's manual. - Use the motor oil grade the manual specifies, since thicker oil can lower mileage. - Combine short errands into one trip so the engine reaches normal temperature. - Remove roof racks and unneeded cargo when you are not using them. ## Trouble codes that often go with it - [P0128](/codes/p0128): Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) - [P0133](/codes/p0133): O2 Sensor Circuit Slow Response (Bank 1 Sensor 1) - [P0171](/codes/p0171): System Too Lean (Bank 1) - [P0172](/codes/p0172): System Too Rich (Bank 1) - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected ## Frequently asked questions ### Why did my gas mileage suddenly drop? A sudden drop most often comes from low tire pressure, a change in driving such as winter weather or more short trips, or a new fault like a stuck thermostat, failing oxygen sensor or dragging brake. Check tire pressure first, then have the codes read if the drop continues over a couple of tanks. ### Does cold weather lower gas mileage? Yes, mileage is usually lower in winter. A cold engine uses more fuel until it warms up, tires lose pressure as the air cools, winter-blend gas holds slightly less energy, and heaters, defrosters and seat heaters add load. Short winter trips are hit the hardest. ### Can low tire pressure affect gas mileage? Yes, underinflated tires flex more and roll with more resistance, so the engine burns more fuel to keep the car moving. They also wear faster and handle worse. Check pressure monthly when the tires are cold and set it to the number on the driver door jamb label. ### Can a bad oxygen sensor cause poor gas mileage? Yes, the oxygen sensors help the computer fine-tune how much fuel the engine gets. A slow or failing sensor can make the engine run richer than needed. It often sets a check engine light with codes such as P0133, though mileage may drop before the light comes on. ### How much does it cost to fix bad gas mileage? Often nothing, if tires or driving habits are the cause. An oxygen sensor or thermostat usually costs 150 to 450 dollars at an independent shop in 2026, spark plugs about 150 to 450, and a brake caliper repair or wheel alignment about 100 to 650 dollars. ### How do I calculate my gas mileage? Fill the tank completely and reset the trip meter. Drive normally until your next fill-up, fill it completely again, and divide the miles driven by the gallons added. Repeat over two or three tanks for a reliable average, since the car's own display can be slightly optimistic. ### Can a bad thermostat cause poor gas mileage? Yes, a thermostat stuck open keeps the engine running cooler than normal, so the computer adds extra fuel as if the engine were still warming up. The temperature gauge usually reads low, the heater may feel weak, and code P0128 often appears. Replacing it typically restores mileage. ### Do fuel additives improve gas mileage? Generally no, most fuel additives do not measurably improve mileage in a healthy engine. Fuel injector cleaners can help if deposits are actually causing a problem, but they will not fix low tires, worn plugs or a sensor fault. Using quality gasoline with detergents is usually enough. ### Does using AC lower gas mileage? Yes, the AC compressor takes power from the engine, so mileage drops while it runs, most noticeably in city driving and on hot days. At highway speeds, open windows add drag too. Using the AC normally is fine, but expect lower mileage in summer heat. ### Will new spark plugs improve gas mileage? They can if the old plugs were worn or causing misfires, because a strong spark burns fuel more completely. If the plugs are still in good condition, new ones will make little difference. Check the replacement interval in your owner's manual before assuming plugs are the cause. ## Sources - [fueleconomy.gov: Keeping your vehicle in shape](https://www.fueleconomy.gov/feg/maintain.jsp) - [fueleconomy.gov (US DOE and EPA)](https://www.fueleconomy.gov/) - [Car Care Council: Car care guide](https://www.carcare.org/car-care-guide/) ## Other car problems - [Why is blue smoke coming from my exhaust?](https://diagxcar.com/symptoms/blue-smoke-from-exhaust) - [Why does my car smell like something is burning?](https://diagxcar.com/symptoms/burning-smell-from-car) - [Why does my car crank but not start?](https://diagxcar.com/symptoms/car-cranks-but-wont-start) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) - [Why does my car jerk when I accelerate?](https://diagxcar.com/symptoms/car-jerks-when-accelerating) - [Why is my car leaking oil?](https://diagxcar.com/symptoms/car-leaking-oil) - [Why does my car lose power when accelerating?](https://diagxcar.com/symptoms/car-loses-power-when-accelerating) - [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle) - [Why does my car make a humming noise while driving?](https://diagxcar.com/symptoms/humming-noise-while-driving) - [Why are my brakes squeaking?](https://diagxcar.com/symptoms/squeaky-brakes) ## Repair costs - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 - [Thermostat replacement cost](https://diagxcar.com/cost/thermostat-replacement): $150-$450 - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 - [Brake caliper replacement cost](https://diagxcar.com/cost/brake-caliper-replacement): $220-$550 - [Wheel alignment cost](https://diagxcar.com/cost/wheel-alignment): $100-$250 - [Camshaft position sensor replacement cost](https://diagxcar.com/cost/camshaft-position-sensor-replacement) - [Exhaust manifold gasket replacement cost](https://diagxcar.com/cost/exhaust-manifold-gasket-replacement) - [Fuel injector replacement cost](https://diagxcar.com/cost/fuel-injector-replacement) - [Knock sensor replacement cost](https://diagxcar.com/cost/knock-sensor-replacement) ## How long parts last - [How long do oxygen sensors last?](https://diagxcar.com/how-long/oxygen-sensor): 60,000-100,000+ miles - [How often should you change your spark plugs?](https://diagxcar.com/how-often/change-spark-plugs): 30,000-100,000 miles - [How often should you replace your air filter?](https://diagxcar.com/how-often/replace-air-filter): Every 15,000-30,000 miles - [How long do tires last?](https://diagxcar.com/how-long/tires): 40,000-60,000 miles - [How long do hybrid batteries last?](https://diagxcar.com/how-long/hybrid-battery) ## Trouble codes that lead here - [P0021 "A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 2)](https://diagxcar.com/codes/p0021) - [P0108 Manifold Absolute Pressure/Barometric Pressure Circuit High](https://diagxcar.com/codes/p0108) - [P0112 Intake Air Temperature Sensor 1 Circuit Low (Bank 1)](https://diagxcar.com/codes/p0112) - [P0118 Engine Coolant Temperature Sensor 1 Circuit High](https://diagxcar.com/codes/p0118) - [P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control](https://diagxcar.com/codes/p0125) - [P0130 O2 Sensor Circuit (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0130) - [P0132 O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0132) - [P0134 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0134) - [P0151 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)](https://diagxcar.com/codes/p0151) - [P0153 O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)](https://diagxcar.com/codes/p0153) - [P0155 O2 Sensor Heater Circuit (Bank 2 Sensor 1)](https://diagxcar.com/codes/p0155) - [P0308 Cylinder 8 Misfire Detected](https://diagxcar.com/codes/p0308) --- Source page: https://diagxcar.com/symptoms/poor-gas-mileage Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.