Hello,

Unless you have specific reason for poor fuel mileage, like an oxygen sensor triggering rich air fuel mixture etc., you can do an engine tune up to restore the efficiency and performance.

If your check engine light is illuminated, get it scanned and retrieve trouble code(s), that could help determine if there is anything needs to be addressed.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Typically, a tune up will consists of oil and filter change, replace air filter, replace fuel filter (if applicable), change spark plugs, clean mass air flow sensor and throttle body etc.

Here is a helpful guide on tune up:

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Please let me know of any questions.
Thank you.