When our rescue moved in, he vomited on every car ride. It wasn't motion sickness, it was anxiety. Sometimes he'd toss it a few times during a single outing. I took him on car rides as often as possible and tried all the suggestions in an attempt to help him; nothing resolved the problem completely. Some trips he did okay, but on others he would get anxious.
The eventual cure was a long road trip. The first day we drove for 12 hours - after half of the day it seemed to hit him that this wasn't too bad after all. Then the second day (which was also long at 14 hours)), he laid down and slept. It seems he just let it go upon realizing that car rides didn't hurt.
Five years later, he jumps in all excited to go and then settles in for a sleep. He's very relaxed in any vehicle. He loves travel and RVing.