This is the second time my gas mileage has gone from 40 to 30 after having my oil changed at Jenkins Hyundai of Ocala. I only have my oil changed there after having a bad experience with a quick oil change business not putting the cover that protects the oil pan back on properly. I always replace the air filter and the cabin air filters myself rather than paying the outrageous prices the dealership wants. i wonder if they are sabotaging my car because I don't accept their suggestions for repairs.

 

Call service desk at dealership and ask what they do when they do an oil change.
Then see if that is what they did. Check air filter? Move the MAF?

Don't know in these cases and very hard to prove.

 

Maybe they use a heavier weight oil, 30 instead of 20 perhaps?