I have a motorcycle and we do the biker wave thing, easy because we're sharing an experience.
I've noticed that Subies and Stangs do a wave/head nod thing.
I see Aveos in my area and I'm trying to start an Aveo wave and only 2 people have acknowledged it. So next time you see an Aveo in in your badass Aveo, wave. We're a rare group now.