Most dogs, given the chance, joyously hang their heads out of a car window and love going for rides. Some, however, either get car sick and nauseated, or are frightened by being in a car. Dogs drool for many reasons, such as nausea and stress. Some dogs, of course, naturally drool and slobber excessively because they have loose jowls. If a dog only drools when in a car, it's probably uncomfortable about the situation.
Dogs who never got used to riding in a car as puppies, or those who only get in a car in order to go somewhere unpleasant such as the veterinarian or groomer, may be frightened or stressed or associate being in a car with bad experiences. Drooling in the car is a sign of stress, especially when accompanied by trembling, flattened ears and a tucked tail. Some dogs are prone to motion sickness, especially as puppies. If they feel ill or vomit every time they get in a car, it can make them stressed--it becomes a vicious cycle and even if they grow out of motion sickness, they continue to drool in the car.
If you're not sure whether the drooling is simply because he is feeling nauseated, or because he is anxious about car rides, ask your veterinarian for medication to quell nausea and motion sickness. Nausea is often followed by vomiting, especially on longer rides--the time it take before a dog vomits varies. If your dog never vomits, but trembles, drools and looks afraid, it's probably more anxious than sick.
Panting is another sign of anxiety, often accompanied by drooling. This can also be a symptom of overheating--dogs are less tolerant of heat than humans are, especially if they have heavy coats and short muzzles. Remember an enclosed car can heat up to lethal levels within minutes; make sure your dog isn't panting and drooling because of heat. Dogs who are feeling nauseated and about to vomit usually keep their jaws tightly closed while saliva dribbles from the corners of their mouths. Nauseated dogs may also lick nearby surfaces, such as the car seat or window glass.
If anxiety is making your dog drool, start slowly and desensitise her to the car. Begin by giving her a treat or playing a brief game next to the car with the door open. Do the same inside the car with the engine off--you may even try feeding her meals in the car. Progress to the engine running, and then very short drives down the street. If your dog enjoys walks, get in the car, drive one block, then attach the leash and go for a walk. Replacing negative associations with positive ones works very well, although it can take some time. The safest way for a dog to ride in the car is secured in a seat belt or in a crate. If your car-sick dog or puppy is used to a crate and you have room for one in your vehicle, line it with newspaper before putting the dog inside. If it vomits, at least it's easy to clean up.
If the drooling is a new behaviour, in or out of the car, take the dog to a veterinarian. More serious causes, such as poisoning, an infected tooth or a splinter can also cause drooling.