WE know the model of the car, and the gas tank capacity..assume that like most people starting a long trip, they filled the car just before they left. We have an estimate of the MPH the car gets on the highway, and thus we can figure otu the farthest they could have driven before refueling, and everyone pays with a credit crad these days..since their cards haven't been used, one has to strongly suspect foul play, or an acident. Sadly, I fear the first option..
Initial reports indicated that they were spotted at a gas station but now the authorities in Virginia are denying them.
http://www.wric.com/Global/story.asp?S=5795655
My guess is that if they made this trip several times a year that they usually left at the same time, stopped at the same places for meals and to fill up the tank.