In all my time living in the south I've literally never heard this.
It's more of a Texas/interior South/non-coastal Deep South thing. It's not as prevalent on the Atlantic Coast. See that map that someone posted above.. I'd say that's reasonably accurate.
No one does this in VA (and not just NoVA, but the "Southern" parts of VA too). Fairly rare in NC too (particularly the coastal areas).. the dividing line in that map above puts it at somewhere between SC and GA, which is consistent with my experience.
Nah, no one here in this part of Texas says "Coke" ever, and there's Dr. Pepper everywhere.
I don't know what to tell ya man. Maybe you just don't hang out around a lot of native Texans? What I can say is that pretty much every map I've ever seen on this includes almost the entire state in the "Coke" zone. The map that was posted above, the map that was in that Upshot
NYT dialect quiz that made the rounds several years back.. heck, go to google images search and do a search for "coke soda map" and you'll see basically every single one has all of TX in it. I've lived in TX as well, and that was my experience (at least with people that grew up there).
Not that it matters or anything, but I'm gonna go ahead and go with the data on this, over your anecdotal post with no data. :shrug:
Poll
