Remember that there wouldn't just be driving time, there would be time to leave the body, although if there was snow, one would just need to find a quiet stretch of highway or a river.
Somewhere with woods. Somewhere he had passed before, somewhere plowed or passable at that time of night and weather. To me, this all argues for a remote stretch of highway north of Ogden, possibly north of the Utah line. It's still interstate but very remote and quiet at that time of night. They would likely have driven this stretch either on the move down or on visits north. It's only a couple of hours to the state line, and a couple hours back. There are also numerous canyons in between that would be quiet and remote yet passable - Huntsville canyon.
It's possible that they went further into the Oquirrh mountains to the west of West Valley City, but these areas would be less likely to have been plowed, and too much in the direction of where he claimed they camped. I don't believe for a minute that the police would have been told the general direction truthfully.
