FUEL ECONOMY HELP · PHOENIX, AZ
Why Is My Gas Mileage Getting Worse? Causes and Fixes
Updated October 2026 · Written by the techs at The Junkie Garage
The short answer
Fuel economy drops slowly, so the decline is easy to dismiss until the fill-ups start stinging. The causes split into two groups: things stealing fuel outright (a dragging brake, a lazy sensor, underinflated tires) and things changing how the car is driven (route changes, more idling, cargo). Tracking a few tanks and checking the simple items first usually finds the culprit without a shop visit.
Cause 1: Tires — the cheapest check
Tires several psi under specification increase rolling resistance enough to cost one to two percent of your mileage per tire's worth of underinflation — and Phoenix heat hides slow leaks poorly, since pressure swings with temperature. Check all four cold, monthly, against the placard inside the driver's door (not the tire sidewall number). Misaligned wheels that scrub sideways do the same damage; if your tires are wearing unevenly, the alignment is suspect.
Wheel alignment costs, explainedWhen alignment actually saves you money.Cause 2: Sensors and filters
A lazy oxygen sensor lets the computer run the engine richer than needed — often silently, sometimes with a check engine code. A dirty mass airflow sensor skews the fuel math the same direction. A severely clogged engine air filter chokes efficiency at full throttle. All are inexpensive relative to what they cost you at the pump each month, and all show up on a basic diagnostic scan.
Cause 3: Dragging brakes and other mechanical drag
A caliper that doesn't fully release, a seized parking brake, or worn wheel bearings make the engine push against a hidden brake. The clues: one wheel hot to the touch after a drive, a faint burning smell, and faster pad wear on one corner. This one matters beyond mileage — a dragging brake is a safety and cost item that only gets worse.
Cause 4: Phoenix reality — heat, AC, and idling
Some of the drop isn't a fault at all. Running the AC nearly year-round costs real fuel; extended idling in drive-through lines and school pickup consumes fuel for zero miles; and summer gas blends plus 115-degree heat measurably cut economy. If your drop coincides with summer, compare the same months year over year before assuming the car changed. We don't do engine repair — our specialty is car AC and community support — so use this guide to ask any shop the right questions.
How we're redefining community supportSee how The Junkie Garage helps Phoenix families stay on the road.Frequently asked questions
How much can a check engine light code cost me in fuel?
Depending on the fault, real losses run from a few percent (a lazy sensor) to 10-15% or more (a rich-running condition from a failed sensor or misfire). A scan costs little and often pays for itself in a month.
Does AC use more gas than driving with windows down?
At highway speed, yes — the drag from open windows roughly matches or exceeds AC load at most speeds, and in Phoenix the AC is non-negotiable anyway. The bigger fuel cost is idling with the AC blasting while parked.
Will premium fuel improve my mileage?
Only if your engine is designed for it. Higher-octane fuel in an engine that calls for regular does nothing measurable for economy — the drop you're noticing has a mechanical or behavioral cause.
How do I figure out my real mpg?
Fill up, reset the trip meter, and at the next fill divide miles driven by gallons pumped. Three consecutive tanks gives a reliable baseline — comparing that to your car's original rating shows how much you've actually lost.
The Junkie Garage is redefining community support
The Junkie Garage is a community shop in Phoenix, AZ. More help from the main site: