Speaking as a cycling, motorbiking, ex-Beetle driving, Motorhome driving person, I find it all very confusing.
When I'm in my car (currently an old, battered Fiesta, so there's no "owner waving" going on there), I often find myself accidently nodding to (motor)bikers. I'm guessing they don't notice, or just assume I've got a twitch. I've also found myself once or twice waving to Beetle drivers, mostly because I forget I haven't owned one for several years.
When I'm in the motorhome, I have to remember to wave to other motorhomes. A nod or a smile probably wouldn't be recognized.
When I'm on the motorbike, I nod at all other motorbikes, mostly because if I waved, I'd probably fall off.
When I'm on the pushbike, I'm almost always too out of breath to say anything. Although as I tend to just cycle around the countryside, I don't often see many other cyclists. The ones I do see usually wave at me as they overtake me (red in the face, puffing away... That's me, not them).
I'm finding it increasingly difficult to remember what clique I'm in, and when. I think I might just start waving to everyone all the time.