I drove with my cat in the car from Denver to Seattle and it worked out very well, of course she is a very calm cat. I put a covered cat box on the floor in the back. I had a cat carrier with comfy towels in it for her to lie down on and I got a large rodent water bottle and hung it on the door for her so she didn't get dehydrated. I spent some time before the trip teaching her how to use the water bottle. I drove a sedan and she found her comfy spot on the back window sill.
Before any door EVER got opened I made sure that the leash was attached and that I had a great hold on it if I was taking her out or that the leash was tied to something if she was not getting out (when I filled the gas tank, etc). I would also recommend getting her microchipped prior to the trip just in case.