Why Is My Gas Mileage Getting Worse?
- Tyler Ellis
- Dec 2, 2025
- 4 min read
If you’re filling up more often and nothing else in your routine changed, your car is telling you something. People ask “Why Is My Gas Mileage Getting Worse?” because MPG loss can creep in slowly—then suddenly you realize you’re burning money every week.
Fuel economy isn’t just about engine health. It’s the combined result of airflow, fuel delivery, combustion efficiency, rolling resistance, and driving load. When one of those gets worse, your MPG usually follows.
This post breaks down the most common causes, how to narrow them down, and what fixes typically solve the problem for good.
Why Is My Gas Mileage Getting Worse? What Causes This Problem?
1) Underinflated tires (the simplest MPG killer)
Low tire pressure increases rolling resistance. Even a small drop can hurt mileage, and if one tire is low, it can also cause pulling and uneven wear.
Clues:
Tire pressure light on
Vehicle feels slightly sluggish
Tires look “soft” on one corner
Uneven tread wear
Fix:
Inflate to the door-jamb spec (not the tire sidewall)
Inspect for slow leaks if it keeps dropping
2) Dirty engine air filter or restricted airflow
If the engine can’t breathe properly, it can run less efficiently. Some vehicles compensate well, but a restricted intake still adds load.
Clues:
Sluggish acceleration
Worse MPG over time
Filter hasn’t been checked in a long time
Fix:
Inspect and replace the air filter as needed
Check intake boots for cracks and clamps for leaks
3) Failing oxygen sensor or fuel trim issues
Oxygen sensors help the car adjust fuel mixture. When they get slow or inaccurate, the engine can run richer than necessary.
Clues:
Check engine light or stored codes
Fuel smell in exhaust
Rough idle or inconsistent power
MPG loss without obvious drivability symptoms (yes, that happens)
Fix:
Scan live data and fuel trims
Replace faulty sensor(s) only after confirming with data
4) Misfires, worn spark plugs, or weak ignition coils
Poor combustion = wasted fuel. Even mild misfires or weak spark can lower MPG before the problem feels dramatic.
Clues:
Rough idle or hesitation
Shaking under acceleration
Check engine light (sometimes flashing if severe)
MPG drop is noticeable
Fix:
Replace spark plugs at the correct interval
Test coils and correct misfire causes (don’t just swap parts blindly)
5) Sticking brake caliper or dragging brakes (sneaky and common)
If a brake is dragging, the engine has to work harder constantly, like you’re driving with a parachute.
Clues:
Vehicle feels sluggish
Wheel smells hot after driving
One wheel has heavy brake dust
Mileage drops noticeably and stays down
Fix:
Inspect caliper slides/piston movement
Repair sticking calipers and confirm no drag afterward
6) Incorrect alignment or worn suspension causing tire scrub
If alignment is off, tires scrub the road instead of rolling freely. That hurts MPG and destroys tires.
Clues:
Car pulls to one side
Steering wheel off-center
Uneven tire wear (inside/outside edges)
MPG slowly worsens
Fix:
Inspect suspension parts (alignment won’t hold if parts are loose)
Perform alignment once the vehicle is tight
7) Fuel system issues (injectors, fuel pressure, contamination)
Leaking injectors or incorrect fuel pressure can cause rich running and MPG loss.
Clues:
Long crank starts
Fuel smell
Rough running on cold start
Poor throttle response
Fix:
Fuel pressure testing + injector evaluation
Correct only what the tests confirm
8) EVAP purge valve problems (often worsens MPG and drivability)
A purge valve stuck open can cause a rich/lean imbalance at idle and affect economy.
Clues:
Rough idle after fueling
Hard starting right after filling up
EVAP-related codes
Fix:
Test purge function and replace if failing
9) Driving load changes (tires, roof racks, cargo, habits)
Sometimes the car is fine and something else changed:
New aggressive tire tread pattern
Roof rack/roof box
Heavier loads
More stop-and-go driving
Frequent short trips (engine never fully warms up)
Clues:
MPG dropped right after a change
No warning lights or drivability symptoms
Fix:
Remove extra drag/weight when possible
Compare MPG over consistent routes and conditions
How to Fix It? (Simple, Fast Path)
If you’re asking “Why Is My Gas Mileage Getting Worse?”, here’s the clean troubleshooting order:
Step 1: Check the easy wins first
Tire pressures (cold)
Air filter condition
Any obvious dragging brake smell/heat
Recent changes (tires, rack, route, habits)
Step 2: Check for codes and fuel trim behavior
Even if the check engine light isn’t on, there can be stored or pending data.
Step 3: Inspect the “silent killers”
Brake drag
Alignment issues
Aging plugs/ignition components
Oxygen sensor response rate
Vacuum leaks
If you want the quickest accurate answer without guessing, schedule an MPG/driveability inspection at Round Rock Auto Center.
How We Diagnose MPG Loss (Without Guessing Parts)
At Round Rock Auto Center, we approach MPG complaints like a system efficiency test:
Confirm the complaint and driving conditionsWe verify if it’s city-only, highway-only, short-trip heavy, or mixed.
Scan for codes + check live dataFuel trims, O2 sensor response, misfire counters, coolant temp behavior, and load calculations.
Inspect maintenance basicsAir filter, spark plugs (age/condition), tire pressures, and any obvious restrictions.
Check for brake drag and wheel resistanceHeat patterns, pad wear, caliper movement, and wheel bearing resistance when needed.
Inspect for vacuum/EVAP issuesEspecially if MPG loss is paired with rough idle or hard starts after fueling.
Recommend only proven repairsThe goal is to restore efficiency without spending money on “maybe” parts.
Why Act Now
Bad MPG isn’t just “spending more at the pump.” It often means something is inefficient or failing:
Dragging brakes can cook pads and rotors
Misfires can damage catalytic converters
Rich running can foul plugs and oxygen sensors
Alignment issues destroy tires faster
Small issues often become bigger repairs over time
If your question is “Why Is My Gas Mileage Getting Worse?”, it’s usually cheaper to solve early than to wait.
Get Your MPG Back Where It Should Be
If your fuel economy has dropped, we’ll find out whether it’s tires, alignment, brakes, sensors, ignition, or fuel delivery—and fix the root cause so you stop burning money every week.
Schedule an inspection with Round Rock Auto Center today: https://www.roundrockautocenter.com
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