Congrats on your new ST!
Per your Owner Guide:
Calculating Fuel Economy
Do not measure fuel economy during the first 1000 miles (1600 kilometers) of driving (this is your engine’s break-in period); a more accurate measurement is obtained after 2000 miles - 3000 miles (3200 kilometers - 4800 kilometers). Also, fuel expense, frequency of fill ups or fuel gauge readings are not accurate ways to measure fuel economy.
I hope this helps clarify things.
Meagan