My guess was we see the first UTV's through check 5 around sun rise which is around 6am. Even at that your looking at a 39mph average. A single bottle neck, stuck in a silt bed or any mechanical issues in the dark and you can tack on several hours.
I'm going to guess the average speed for the winning UTV is in the low to mid 30's. I have been wrong before, but I think averaging anything over 40 is going to be really tough. Like I mentioned in a previous post. I think the Check Point time limits are going to a huge factor in this race. Lets hope at the drivers meeting they announce that they are extending the overall time limit and extending a few check point times.