Susan was 5'4.
I'm going to go out on a limb and say I don't think there is even a
chance this is Susan.
Unless he had an accomplice that he met, who took her body the rest of the way.
I-80 actually gets CLOSED frequently during the winter because the snow and ice get so bad on this route through Wyoming. They have gates that they simply shut. One of them is right at Laramie. Given the weather in Utah at that point, there is a very good chance I-80 was closed or very ugly. I do know there WAS an ugly bus/semi accident on December 6th, which killed the bus driver. This was due to ice.
Even with 24 hours, if he went when he rented the vehicle...I do not think he could have driven anything without 4 wheel drive or chains up there. Even with that, you would end up going about 20MPH. Which would turn that 6-8 hour drive (an estimate depending on the route you take, without weather conditions slowing you down) into FAR longer.
You would be taking a HUGE unneeded risk by going to Wyoming. Getting stuck, crashing, having to go 20MPH the whole way. I think there is about a 1% chance this is Susan and if it is, I am 99% sure he had another person helping him. I firmly believe Susan is in Utah or one of the
other surrounding states, or there is someone else going down with Josh.
Here are a couple things about the weather in December 2009, I'm sure someone with more time can find something specific to December 6th/7th when she went missing or December 9th/10th when he rented the car. But this is a pretty good general idea. I will look more later if no one else has.
Dec 11 - Winter storm watch, travel treacherous -
http://www.pinedaleonline.com/news/2009/12/WinterStormWatchforw.htm
Dec 26 - 1-80 closed - picture at Laramie:
http://www.pinedaleonline.com/news/2009/12/RoadWeatherandTravel.htm