Scotty, 

I know you get asked this question all the time but I have a 2016 Mustang GT. After getting it tuned, adding a CAI and exhaust soon after buying the car, I was getting 18.9 miles per gallon steadily for years. The car has 80k miles on it. The car Average MPG dropped by overnight to 16.9. It’s like in one day, it just dropped and stayed at 16.9 for years. Then it dropped again to 15.2. I keep the car maintained very well. I do all the work on it myself, and use the best oil, etc out there. The tires are full and I drive it the same way I always have driven it. I always have the AC on because it’s always hot in south Louisiana, so there is no change there, but for some reason the mileage just keeps dropping. It literally seems like it it drops average MPG like Ford programed it to do this. Not saying Ford did that, but it was steady at 18.9 average for years, then after a few yers it suddenly went to 16.9, then after another few years it hit 15.2. There was no “in between.” When gas was cheaper, it wasn’t much of a bother, but I have it tuned for 93, so I have to buy the more expensive stuff. Since Biden took office, inflation and gas prices are starting to hurt. 
I don’t know if an injector, oxygen sensor, Mass air flow sensor is bad. I cleaned it with mass air flow cleaner multiple times, ran Seafoam through it multiple times. I have no codes. 
I don’t know what to do next. I’d hate to start replacing sensors, etc. just cause.  Any help is appreciated. Thanks for all you do for use car guys (and gals) out there