To work out what MPG you are really getting, please provide the cost of the fuel, how much you spent on the fuel, and how far it took you.
Gastritis is found among people who take Prednisone, especially for people who are female, 60+ old , have been taking the drug for < 1 month, also take medication Zometa, and have Rheumatoid arthritis. This review analyzes which people have Gastritis with Prednisone. It is created by eHealthMe based on reports of 225,254 people who have side effects when taking Prednisone from FDA, and is updated regularly.
You don't say what you are being treated for, but sometimes, say with an allergic or similar type rash that has a secondary infection, we give prednisone for the underlying reaction causing the rash and antibiotics for the secondary infection; there are other reasons to combine the two such as extreme throat swelling with strep or respiratory infection; without more info, however it is speculation.