Your gas mileage dropped

Step 0: is the drop real?

Fuel economy varies tank to tank for reasons that have nothing to do with the car: how the pump shut off, whether that tank included a highway run, the temperature, a week of short trips. A single bad tank is noise. Two bad tanks are still probably noise.

• Compare rolling averages, not tanks. Five tanks against the previous five. If the difference is under about 5%, you are chasing your own measurement error.
• Compare against the same season last year, if you have the history. This one check kills most winter panics outright.
• Do not trust the dashboard for this. Trip computers estimate fuel rather than measure it and tend to read optimistic — a drop shown there could be a recalibration or a reset. Hand-calculated MPG from the pump is the reference; the method is in the MPG guide.

Free, five minutes

• Tire pressure, cold, to the doorjamb placardPressure falls about 1 psi per 10°F drop in ambient temperature, so autumn quietly deflates every tire you own. The Department of Energy puts correct inflation at up to about 3% better economy — modest, but free, and it also buys tire life and better handling. Use the placard number, not the maximum printed on the sidewall.
• Empty the carExtra weight costs fuel, and roof racks and cargo boxes cost far more than their weight because of aerodynamic drag. If a roof box has been up since the summer trip, that alone can explain a several-percent loss at highway speed.
• Audit your own drivingNew job, new route, more stop-and-go? Cold-start-heavy short trips are the most fuel-expensive miles there are — the engine spends much of the trip below operating temperature. Ten minutes of daily idling to warm up is worth several percent over a month.
• Check for a dragging brakeAfter a drive, carefully compare wheel temperatures. One noticeably hotter wheel means a sticking caliper or a seized slide pin — a real and commonly missed fuel-economy killer that also destroys pads and rotors.
• Confirm nothing is stuck onA rear defroster left on, heated seats, and on some vehicles a permanently engaged AC compressor for the defrost mode. Small individually; measurable together.

Cheap, under $50

• Engine air filterLook at it. A visibly clogged filter on an older engine costs measurable economy; on modern fuel-injected engines the effect is smaller than folklore suggests, but a filter that has been in for 40,000 dusty miles is due regardless.
• Fuel quality and the station you changed toDid the drop coincide with switching stations? Contaminated or water-affected fuel happens. Note that many US regions sell winter blends containing more ethanol, which carries less energy per gallon — several percent, seasonally, with no fault at all.
• Spark plugs, if they are past intervalWorn plugs cause incomplete combustion and, eventually, a misfire. If the log says they were done 90,000 miles ago, this has moved from suspect to overdue.
• Tires themselvesNew tires — especially aggressive all-terrains or winters — commonly cost several percent versus the ones they replaced. A larger overall diameter also makes the odometer under-report distance, which shows up as an MPG drop that is partly arithmetic rather than fuel.

Scan for codes

A basic Bluetooth OBD-II reader costs less than one tank of fuel, and most US auto parts stores will pull codes for free. Do this before replacing anything.

Shop-level causes

• Dragging brakes — the hot-wheel test above; confirm on a lift.
• Alignment — a car fighting itself scrubs speed continuously. Uneven tire wear is the visible symptom.
• Failing thermostat — cheap part, meaningful effect, easy to confirm with a scan tool's coolant temperature reading.
• Clogged catalytic converter — usually accompanied by lost power and, eventually, a code.
• Fuel injectors — leaking or poorly atomising injectors run the mixture rich. Uneven fuel trims across banks are the giveaway.
• Carbon on the intake valves — a known issue on many direct-injection engines past roughly 80,000 miles, since the fuel spray no longer washes the valves.
• Transmission not locking up — a torque converter that will not lock, or an adaptive transmission stuck in a bad learned state, quietly costs highway economy.

The seasonal pattern

Before concluding anything is broken, check whether the calendar explains it. The US Department of Energy notes that short trips in cold weather can cut fuel economy by roughly 10–20% in city driving. Contributing factors:

• Longer warm-up. A cold engine runs an enriched mixture and everything from oil to differential grease is thicker.
• Denser air. Better for power, worse for drag.
• Winter fuel blends. Formulated for cold-start volatility; in many regions, slightly less energy per gallon.
• Winter tires. Softer compounds and aggressive tread cost rolling resistance.
• Accessory load. Defrosters, heated seats, headlights on at 4pm.
• Idling to warm up. Infinitely bad economy while stationary — 0 miles per gallon burned.

If your drop starts each November and recovers each April, the car is fine. This is exactly the pattern that only a multi-year log can show you — and the reason it is worth logging through the boring years.

If it's an EV or hybrid

• Cold hits harder. Battery chemistry is temperature-sensitive and cabin heat comes from the pack rather than from waste engine heat. Winter range losses of 20–30% are commonly reported; preconditioning while plugged in recovers much of it.
• Charging losses are real. The kWh your meter bills is more than the kWh that reaches the battery. Efficiency computed from the wall is the honest number for budgeting.
• Hybrids are especially short-trip sensitive. The engine cannot shut off until the catalyst is warm, so a five-minute commute is close to a worst case.
• Check tires and drag first anyway. The physics above applies identically; EVs are just quieter about it.

Questions

Sources

• US Department of Energy / EPA, fueleconomy.gov — cold-weather effects on fuel economy and the effect of tire inflation. fueleconomy.gov

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