When Doogie and Vinnie initially decide to try to get to Chicago by 8 am the next day, it is already bright outside in Los Angeles. Even allowing for the very unlikely possibility that is only about 6 am, that gives them 24 hours to reach Chicago 2000 miles away (since Chicago is two time zones ahead of Los Angeles). This would require them to maintain an average speed of more than 80 mph for 24 hours straight - an extremely unfeasible goal, even disregarding the problems along the way.