Check the computer for a knock sensor code. If the code sets (a knock sensor code will not turn on the check engine light) then it will kick off the boost, retard the timing and BAM will kill gas mileage.

Techs hate doing them, so many won't tell you that the code is there. You'll have to ask specifically to get it. Then, be prepared to dish out some coinage, 'cause the sensor aint cheap, neither is the labor.

Set your timing to +2° advanced from base spec and run some injector cleaner through the system. BG 44K works good.

Also remember, many places run a winter blend of fuel to combat the brown cloud. You'll run across several different blends as you drive cross country in the winter. Any cross country trip in the winter will result in less fuel mileage vs. doing the exact same trip in the summer.

Enjoy the S/C X!