Fuel Mileage Drop

We all told you to check your tire pressure on the other thread when you asked all these questions.

You have a real mess of tire mix. 3 are not LRR tires. Very unsafe in this car.

The battery voltages you posted are not correct. You are missing the most important step the load test. You applied the headlight load test in the wrong mode. 12.8 in load is impossible. Re-test.

Well known for years now you will take a TEMPORARY mpg hit when you have the car serviced. It sits at idle for long time and 12 volt battery stressed, many short trips accumulated etc. Many reasons. To fix that reset mpg meter on dash & take a 10 mile drive on highway at 50 mph.