Travelling in a car is much safer for the dog if it is confined to a crate or with a proper car guard. The time spent training the puppy to accept and like being in a crate will pay dividends in this instance. Plan your journey, how long are you going for? What will you need to take with you (food, water, bowl, leads, collars, treats etc)?
One thing to do would be to have a collapsible dog bowl and a bottle of water always in the car, in this way you are always prepared for a journey and your dog will be happy and healthy. A dog should never be left in the car unattended. Temperatures rise quickly as dogs have difficulty regulating body temperature this can be fatal. When travelling in the car, plan, think of safety and always avoid being unattended.