I'd think that new tires = greater traction = less mileage
but my 2002 3.1L Buick Century V6 went from ~32mpg to ~27mpg hiway. Can new tires really make it drop that much? I had old tires that would barely pass inspection, and then just put on new all season Yoko Avid TRZs (which are not M+S), before a 200 mile trip. Tire PSI is good. I'd changed the oil before the trip, too.
Another factor was a CEL code of P0442 that had been registered a couple weeks ago: