True motion-sickness will usually require medication to help with the associated nausea. If your dog salivates during rides then its likely more than just not enjoying the car ride.
If however your dog is more fearful or stressed by the experience, they can be helped to overcome some of the negative feelings by the following:
Try to stop off at the park or another favorite walking spot so that your dog is rewarded for “putting up” with the experience.
Try a different car if you think the association could simply be too many negative experiences in the same car
When the reaction is mild or your dog hasn’t been experiencing negative associations with car rides for long, you can use the above points to help them overcome the anxiety associated with car rides. If there is no relief for your dog, please book a training session so the experience can be reintroduced to them, for them to start making positive associations and even come to enjoy car rides.