Have you tried taking Sparkle on short trips to acclimatize her to the car? Gina (of gina's ark, inc.) started taking her cats on ten-minutes drives, then gradually made them longer and longer, and last winter was able to drive with them in her car from Calfornia to New York without incident.

I used to get motion sick a lot, but haven't since the brain surgery (acoustic neuroma removed) that left me half deaf. It's so good to not get motion sick! Tell Sparkle she has my sympathy.

Others have tried Rescue Remedy, which is not as strong, I guess, as a tranquilizer, but may help her relax a bit.