Benadryl is your friend. 1/2 a pill for anything under 20 pounds. 1 full pill up to 60 and anything over 1.5 pills.
Its usually anxiety, not the motion, that makes dogs sick in a vehicle. So settling their nerves is the wisest approach.
Ive been around dogs my entire life, coming from a family of obedience trainers. We had a dog some years ago that would literally hyperventilate during 4th of July and thunderstorms. My wife and I aren't ones for pushing pharmaceuticals so we used benadryl. Works like a charm.