I don't know if this works for dogs, but it works for kids. "Ground" the vehicle.

As a youngster, in the 40s and 50s, I suffered terribly from motion sickness, often passing up long trips with friends because I knew I'd have to ride in the back seat.

A friend of my mother's told her to ground the car, by attaching a chain or metal strap to the frame of the car and letting it bounce on the pavement. This was the same means employed, at that time, by tanker trucks to keep road static from causing an explosion or fire. It worked. When I started driving I realized I never got motion sickness while I was at the wheel. I was "grounded" by contact with steering column and thus the roadway. I still get motion sick unless I drive.

The problem is worsened by riding low in a vehicle, such as a pup might ride on the floor (fumes are worse there, too).

As Sorcha said, there is motion sickness preventative available from a veterinarian. Some friends have a dog who is guaranteed to barf before they are a block from home. They give her the medication about 30-45 minutes before leaving and she sleeps for most of the trip.

Good luck. Mary