Symptoms
- Filling up more often than usual for the same driving
- Trip computer showing lower MPG than normal
- Possibly rougher idle or reduced power alongside it
- No obvious warning lights in some cases
Likely causes
- CommonUnderinflated tiresLow tire pressure increases rolling resistance and can quietly cost several MPG.
- CommonDirty air filterRestricted airflow can push the engine to run richer than needed.
- CommonWorn spark plugsWeak or fouled plugs cause incomplete combustion, wasting fuel.
- PossibleFailing oxygen sensorBad sensor feedback can keep the engine running rich.
- Less commonDragging brakesA stuck caliper creates constant drag the engine has to work against.
What to do
- Check and correct tire pressure on all four tires, including the spare, to the number on the door jamb sticker.
- Check the air filter and replace it if it's dirty or overdue.
- Note your actual driving conditions (more idling, cold weather, AC use) since these normally lower MPG too.
- Get the check engine light scanned for any pending codes even if it isn't illuminated.
- Have spark plugs inspected if they're near or past their service interval.
- Have a shop check for dragging brakes or a failing oxygen sensor if the simple items don't help.
Tools and parts
- Tire pressure gauge
- OBD-II code reader
Engine air filterSearch: “engine air filter for [year make model]”
Spark plugsSearch: “spark plugs for [year make model]”
When to call a pro
If basic checks don't explain the drop, a shop can pull fuel trim data to find a sensor or mixture problem quickly.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.