We recently took a road trip out to the coast of Oregon. It was slightly longer than usual due to a rock slide that closed a highway and forced us to backtrack and take a longer detour route.
Shadowfax is really good in the car, she generally just lays down and sleeps once the car starts moving. She really only gets up and looks out the window when we slow down and pass through a town. This is her after a nice tour through a rest area, so many good things to sniff.
We take car safety seriously, so we have her stay in her harness (it's a RuffWear Harness) while she's in the car. It has front and back clip on spots which makes getting in and out of the car easy, and she can stay leashed to the seat belt but still have some freedom of movement.