For safety purposes, your dog should not be allowed to roam freely in the car. A dog should be taught to sit or lie quietly in the car. Do not allow the dog to stick its head out the window. Debris could damage the eyes and excessive air could damage its lungs. Never transport a dog in the bed portion of a pickup truck.
Cats generally do better if kept inside a carrier.
Have your dog's nails clipped before a trip to prevent scratches and upholstery damage. If you plan to use a crate, insure you acclimate your pet to the crate prior to travel. Start with brief periods of placing your animal in the crate at home then gradually increase the time. Graduate up to placing your animal in the crate while in the car. Be patient and you will see the rewards.