Bottom line: It's like an ice cream cone of smells
Dog's noses contain 300 million olfactory receptors (compared to our measly 6 million), and the part of their brain dedicated to sniffin' is about 40% larger than ours. Simply put, dog's noses are pretty much the equivalent of our eyes, sensory wise.
That means when your window is open and Fido's head is out over the freeway, it's getting a veritable, all-you-can-sniff smorgasbord of exciting new smells flowing rapidly over their noses—which apparently, they really, really dig, as pointed out by author/veterinarian Marty Becker in his book answering common canine queries.
Dr. Janie von Waldburg, a veterinarian from PetCoach, confirmed this theory, adding that "They really do love all the smells coming in...it's like an ice cream cone of smells for your dog."