A gradual decline in fuel economy is frustrating — and it's one of the most common complaints from car owners. The good news: most of the causes are inexpensive to fix. Here's how to diagnose the issue systematically.
Check These First (Free)
Tire Pressure
Underinflated tires are the single most common cause of MPG loss. Each 10 PSI below spec reduces fuel economy by roughly 2.5%. Check all four tires when cold. This takes 5 minutes and costs nothing.
Driving Habits
Hard acceleration, excessive idling, and high-speed highway driving all reduce MPG significantly. If your habits haven't changed, look elsewhere.
Check These Next (Inexpensive)
Air Filter
A clogged air filter restricts airflow and richens the mixture. Pull the filter and hold it to light — if you can't see light through it, replace it. $15–25 part.
Spark Plugs
Worn plugs cause incomplete combustion and lost efficiency. If you're past your service interval, fresh plugs often restore 5–10% fuel economy. This is the most common maintenance-related MPG loss culprit.
Scan for Codes
A check engine light (or pending code even without a light) can reveal: oxygen sensor faults (P0130–P0167), fuel trim issues (P0171/P0172), or MAF sensor problems. These directly affect fuel delivery and economy.
Harder to Diagnose
- Stuck-open thermostat (engine never reaches operating temp)
- Dragging brakes (calipers not fully releasing)
- Failing fuel pressure regulator
- Clogged catalytic converter (check back pressure)
Start with the simple stuff — tires and spark plugs fix the majority of MPG complaints. Shop spark plugs at Texan Supply with free shipping.