There are people out there who will tell you that you shouldn't let your dog stick his or her head out of a car window. That it's unsafe, because your dog could get hurt in a crash, whacked in the face by a flying pebble, or develop an ear infection from letting those big, floppy earflaps blow in the breeze. These people are safety Nazis.
No, dogs deserve to have their heads sticking out of car windows just as much as we deserve to stick our arms out 'em. Just don't drive too fast if your pooch is catching some air, and you'll be fine. Your dog, on the other hand, will be experiencing a form of bliss unknown to humans with complex psyches and pathetic olfactory equipment—a form of bliss best captured in slow-motion.