Which of these will cause a vehicle to use more fuel?

A dirty air filter can significantly impact a vehicle's fuel efficiency. When the air filter is clogged with dirt and debris, it restricts the airflow to the engine. This restricted airflow can lead to a rich fuel mixture, meaning that the engine may receive more fuel relative to the amount of air it gets. An optimal air-fuel mixture is necessary for efficient combustion; when it is off-balance due to insufficient air intake, the engine then works harder to compensate, which in turn leads to increased fuel consumption.

Maintaining a clean air filter ensures that the engine has the correct amount of air, promoting better combustion and fuel efficiency. By contrast, while a rusty engine, carburetor problems, and clogged fuel lines can also lead to inefficiencies, they do not have the same direct impact on the air-fuel mixture and overall operational efficiency as a dirty air filter does. In essence, a dirty air filter creates a situation that demands more fuel usage to achieve an acceptable level of engine performance.