Thanks for the replies.
As 81 3/4 matches neither of the two dimensions quoted from the GM site, I wonder where they get their dimensions from?
The reason I am seeking an answer, as alluded to in my original post, is a tonneau cover manufacturer (Bison, in eastern Canada) has a solid tonneau that I am interested in, that a couple persons have mentioned seems a bit short for the bed it is made for. So, I wanted to have exact dimension prior to having a conversation with their customer service. And, as also mentioned, my newly ordered truck is en-route and not available for measuring.
Thanks again,
Marv