What are the reasons for the sudden decrease in fuel economy in my Audi A4?

I have noticed a significant decrease in the fuel economy of my Audi A4 over the past few months. Although the car is regularly serviced and well-maintained, the mileage per gallon seems to have dropped drastically. I have not made any modifications or changes to the car, so I am wondering what could be causing this sudden decrease in fuel efficiency.

One possible reason could be the air filter in your car. A clogged or dirty air filter can reduce fuel efficiency by as much as 10%. It is recommended to replace the air filter every 12,000 to 15,000 miles.