A sudden drop in fuel economy is your engine telling you something is making it work harder than it should. Sensors, ignition, and tires are the top culprits. Here are the most likely causes ranked by how often they turn out to be the problem.
A lazy O2 sensor reports wrong mixture so the ECM runs rich. Can drop mileage 10-20% before triggering a code.
Restricted airflow or a dirty MAF makes the ECM richen the mixture. $15 air filter or $10 MAF cleaner often gets mileage back.
Worn plugs misfire under load, wasting fuel. Replace per spec - usually 60k-100k miles.
4 tires 5 PSI low costs 2-4 mpg. Free fix. Check monthly.
A dragging caliper costs constant fuel. Feel for a hot wheel after a short drive.
You smell gas inside or outside the car, the check engine light is flashing, or fuel range dropped by more than 30%. A flashing CEL means active misfire that can destroy the catalytic converter and cost $1500+.
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A lazy or biased O2 sensor can cost 10-20% of your fuel economy. Replacement is usually $150-$400 installed and pays back fast.
Most often a lazy O2 sensor, dirty MAF, dragging caliper, or low tire pressure. All are findable with a basic scan and a quick inspection.
Only if your car requires it. In cars designed for regular, premium gives no mileage benefit and just costs more.
Modern fuel-injected cars are less affected than carbureted, but a severely clogged filter still costs 1-2 mpg.
A 10-15% drop in cold weather is normal. A sudden 20% drop in good weather is a problem worth investigating.