Car sickness is just one type of motion sickness. While many puppies and young dogs suffer to some extent from car sickness, most will outgrow it by their first birthday when the area of their inner ear associated with balance is fully developed. Some dogs, however, don't outgrow it until closer to their second birthday, and then there are some dogs, who like some people, just get sick every time they ride in the car. There are things you can do to try to ease their discomfort, but not every technique works for every dog/situation.
Start by having your dog only ride in the car with an empty stomach. Only drive a short distance and build up to longer rides. Don't use the A/C, instead roll down the windows. Make sure your dog is facing forward, either in a crate or using a dog safety harness. Stop frequently and let your dog walk around; this helps them get their "sea legs," so to speak. If your dog begins drooling even before getting in the car because they are anticipating getting sick, try just getting into the car, rolling down the windows, etc. but not going anywhere. Once they can sit in the car without anxiety, build up to turning the car on, and then going a short distance such as just around the block.
If you must take your carsick dog a long distance, being prepared will help. Benadryl works for some dogs to make them drowsy and thus calmer, sleeping off their nausea. For other dogs, Benadryl won't work, but CBD oil or CBD treats will, so that's worth a try. For example, when Ozzie was a puppy and got sick in the car, I discovered that giving him a couple ginger snaps cookies about 30 minutes before a car ride (the Nabisco ones you find at the grocery store!) was sufficient to control his nausea.
Finally, having to bathe your dog after every car ride is no fun. You can try putting a bib on them to soak up the drool. There are even plastic bibs that come with a "spill pocket" on the front that can work to catch drool and vomit (Ozzie had one of those!). Some people just put a t-shirt on their dogs to soak up the saliva. There are carseat covers that are easy to clean and are actually made to cover the seats and protect them from kids that get carsick!
Because dogs that get carsick begin to associate car rides with feeling crappy, you will have to put in some time desensitizing your dog to the car in order to eventually make car rides less of a stressor. For Ozzie, short car rides facing forward with walks built in, ginger snaps, a plastic bib, and time to outgrow his motion sickness, were what it took. Now, he loves car rides and can go any distance without an issue.
As always, if you have questions about your pet's behavior, you know where to find me.
Ozzie loves a good car adventure now, but got very carsick as a puppy.