did you do the core program or the loaner program. if so did you did a CASE relearn? if not you gas millage will stink
how fast were you going? the optimum speed for the good mpg is about 55/60 if you were going 75 the most of the way even with cruise control it would hurt mpgnot so much...on a few road trips that i have taken, i was careful and not to gun it for a full tank of gas. i set the cruise to like 5 or so above the posted limit and just cruised.