Miko used to do this all the time when he was younger ( he even got sick a few times), and I attributed it to being stressed from just being in the car. Unfortunately, we have to travel alot, and I was worried about him but after time and time of taking him in the car, he got used to it, and now he's fine with it. Once we hit the interstate, he lays down and sleeps the whole way
No more excessive salivating or panting...He does get antzy (sp?) when we get off the interstate and are in stop and go traffic, but it doesn't seem to bother him anymore.