Dont worry about it.
When I used to drive my 68 RS around town, id wave at anyone that waved at me. Also, there was a red 67 that used to run around time about the same time I did. I always waved at him and even thought id get to know him so we could chat about our first gens. But the guy always frowned or looked hateful back at me. Never waved at me not once. His problem not mine. Just don't let it get to you. They are simply jealous is all. Just laugh and move on. Best thing you can do is when they flip you the bird is smile and wave at em in a friendly wave.
I used to let it get to me and chase people down when I was younger. No reason to do that. Just wave and smile. Your the better person.
