When I was a kid, my family moved from Pittsburgh to St. Louis. We had two cats, a 10 month old golden retriever, and two kids (7 and 10). It was pretty hectic.

Our cats hated the car! They would meow, hiss, claw, etc. etc. This may sound mean, but our vet gave us a mild tranquilizer to calm the cats during their car ride. It didn't completely knock them out, but they pretty much just slept and sat there quietly in their carriers. A few meows of protest were still heard. I know the word tranquilizer is a bit scary, but what we had was safe and a mild dose. I would definately ask your vet about the options.