Summer formula netted an average 24 mpg, winter came in at a hair over 28 mpg. Both trips were in the same car, 2007 Mazdaspeed3, only change to the car is a C.A.R.B. EOed cold air intake.
That's interesting.... everything I have heard or experienced indicates slightly worse fuel mileage with winter fuel formulation. I take it the CAI was installed prior to your winter-fuel trip? That might explain your improved mpg, and makes for somewhat of an apples-to-oranges comparison.
Also, it's hard to get a very accurate MPG figure with just one fill-up. There are too many variables that can affect exactly how full you get the tank each time.
