2013 Fiesta SE Hatch Manual Trans 1.6 Liter Engine 55,000 miles. Bought brand new. The odometer has consistently reported 37 MPG overall mileage until lately. It is now showing 35 MPG overall MPG for the last several months. Seems a little early for spark plugs. No fuel in the oil. Hardly measurable oil usage between oil changes that I do my self. Mobil 1 From the first time I changed oil. Next oil change is due in several hundred miles. Oil analysis always come back very good. I plan to check the compression at next oil change. I do suspect gasoline quality is heading south in Texas (Shell Regular) - I drove up to Kansas and out west to the Pacific coast last year. Car ran better on gas once I was going west. Even in California. As I entered Texas on the way back I got less pep and poorer MPG. Anything else I should check?