If your vehicle is showing noticeable drop in fuel economy or terrible gas mileage, here are the most likely causes and the parts you'll need to fix each one.
Low tire pressure increases rolling resistance, forcing the engine to work harder to maintain speed. Poor wheel alignment can also cause the tires to drag, significantly reducing fuel efficiency.
Old, fouled, or improperly gapped spark plugs cause incomplete combustion, meaning unburned fuel is wasted out the exhaust instead of making power.
A severely clogged air filter restricts airflow to the engine. While modern computers adjust fuel to match the lower air volume, the engine has to work much harder to pull air in, reducing overall efficiency.
A degraded O2 sensor can falsely read a 'lean' condition (too much air). The engine computer responds by dumping excess fuel into the cylinders to compensate, ruining gas mileage.
The MAF sensor measures incoming air. If it gets dirty, it miscalculates airflow, leading to an improper air-fuel mixture. This is frequently caused by over-oiling aftermarket reusable air filters.
If the thermostat is stuck open, the engine never reaches its optimal operating temperature. The computer thinks the engine is still warming up and keeps it in a fuel-heavy 'cold enrichment' mode.
A seized brake caliper or stuck slide pins will cause the brake pads to constantly drag against the rotor, acting like an anchor. This is often caused by failing to lubricate slide pins during a recent brake job.
An injector that doesn't close properly will leak raw fuel into the intake or cylinder. This drastically lowers fuel economy and can wash protective oil off the cylinder walls.
A restricted exhaust system chokes the engine, preventing it from breathing out. This drastically reduces power and fuel economy, and usually causes the engine to struggle at highway speeds.