You are new here so I will give you the benefit of the doubt that you properly filled your tank and measures the amount of fuel that you used correctly. But we've had some major discussion here on how to actually fill the tank, use the same gas pump from the same gas station and things like that which can affect the mileage calculation. The real concern is that an engine sitting for 2 years can develop rust in the cylinder bores and wear the rings when started back up but that should affect mileage right away. The only unknown is the 2 year old fuel and what affect that would have on your mileage when you burn it. Normally you would lose mileage from the loss of gasoline from evaporation but not counting that loss you could possibly and up with a more concentrated, higher mileage fuel than fresh gasoline would give you.
Stale gas has no more energy in it I saw a car that had 2 year old gas come into the shop. The gas didn't even smell. I had to drain it all. I tried to run it 50/50 in my car and I could tell my car and old Volvo 740 it would run on anything, it didn't like that stale gas. Do a compression check and hook up a vacuum gauge it will tell you tons of info on the car. Check your timing it could have jumped a tooth. That would run and be off just enough check your 02 signal is it working. Lazy 02 wastes gas. Start with basic tune up. Check plugs they could be all fouled out.
I just filled up earlier with 76 premium fuel and drove 88.0 miles, just filled it back up with premium fuel from Exxon and it only took 1.522 gallons. That's what 57.81mpg. Kinda confusing how the regular vs premium doubles the mileage?
That's fuel expansion of a full tank while driving most likely plus too few miles driven. Plus if the tank wasn't full to the top and spilling over you may have had an air pocket at the second filling.
Have you checked your tire pressure since you started driving?
Maybe a blocked oil filter as the engine flushes any sludge from inside the engine causing a load on the pump too?
Filling a small amount at two different stations is not a good measure. Each pump shuts off a little differently, and you could have experienced pumps that shut off very differently. You may have gotten that 57mpg, you may have gotten 30mpg, you (unlikely) may have gotten 70mpg...the shutoff difference need only be a small amount to make a big difference when you're pumping so little.
When filling up the first yesterday, it was all way to the top of theyou fuller neck and car positioned facing downhill both times. That is why I am confused from using regular to premium. I ordered an o2 sensor before I calculated the fuel mileage. Tire pressure at 35. I need the gas mileage as I am a pizza driver.
You need to run like 3/4 of a tank and go back to the same gas station and use the same pump and then put it on the slowest locked setting and when it stops it's done do that 2-3 time then you will have a true mpg figure. Also is this all city stop and go driving? If so it's hard to get the same mpg figures. Ie more hills more stops more idle time.