What can I do to improve poor fuel efficiency?

Hello,

There could be a number of reasons why you are getting poor fuel mileage.
From basic engine tune up to emission or other issues.
Are you experiencing any drivability issues?
Is the check engine light on? If so scanning and retrieving the codes can help with the diagnostics.

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

Many times, doing a tune up can improve overall efficiency significantly. Like replacing the air filter, changing out the spark plugs, cleaning the throttle body and mass airflow sensor etc.

Take a look at this helpful guide:

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

Let me know of any questions.
Thank you.

My car does not seem to have any drivability issue and no codes upon scanning. But as I said mpg is only 12-13. I tried adding those cheap additives available for about $10 at AutoZone, in gasoline which are supposed to clean the injection system but no difference. Should I inspect the spark plugs next? or just change them to see if it helps.

Follow the guide and check the typical items and take the suggested steps.

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

If the plugs are worn, then replacement would be recommended. Be sure to check the air filter as well as the mass air flow sensor.

How can I change the spark plugs myself on this car? I went to a repair shop to get quotes on replacing just spark plugs. He was asking $450+ which I cannot afford.

See attached for the spark plug replacement procedure.
Keep in mind that you will also need to check the air filter and replace it if necessary, also cleaning out the throttle body is also recommended.

https://www.2carpros.com/articles/how-to-replace-a-car-air-filter
https://www.2carpros.com/articles/throttle-actuator-service

See diagrams below.