Even when babying the car, I'm getting about 3mpg less than I did when I was not babying the car previously. Air filter clean, new oil, 27k miles. Only thing I can think of is the spark plugs? I don't have the manual on me...what was the recommended spark plug change interval? I'm completely stock, so should I just go with stock plugs? What are stock plugs? O'Riley's and auto zone both give me different plugs when I put in my vehicle info.