It's not just these trucks. Motorcycle owners almost always wave to each other and I know vette owners do.
In my area, thumbs up and waves are rare though. Most people just keep a low profile. I have gotten a few thumbs up's while driving my 66 Chevelle. The first time was from someone driving a Volvo of all things!
Other Chevelle owners have occasionally gestured to me also.
Don't worry though, even if they aren't waving, their looking! These old trucks are the best looking trucks on the road.
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63 Impala*66 Chevelle SS*69 Chevelle*69 C10 Stepside*71 Cheyenne 20*72 Chevelle
Parts for sale:
http://www.angelfire.com/trek/mr409/items4sale.html