A reasonable speed for the car to travel at for most of the trip would be approximately 65 miles per hour (mph).
I picked this number because it is a common speed limit on many highways in the United States. Additionally, it allows for a balance between efficient travel time and safety. Most highways are designed for vehicles to travel at speeds around this range, accommodating for both faster vehicles and those that may need to drive more slowly for various reasons. This speed also allows for a good estimation of distance covered over the eight-hour trip.