I'm wondering if the "soda" clusters in Milwaukee and St. Louis don't have something to do with "Big Beer" imposing its wishes on consumers.
When I first moved to New England the teacher was talking about whether we could bring "tonic" to the school outing. It scared me a little. I thought "What, we're taking patent medicine now?" but it was soda. That usage is dying out now.
Color me skeptical about some of the distinctions, though. "Carmel" and "Caramel" or "Maynaise" and "Mayonnaise" are the same thing -- until you want to play with somebody's head.
My hometown was actually a weird case. There’s a more detailed version of that map that I’ve seen, and running up the Illinois River Valley from Missouri into Central Illinois is a finger of area where it’s “sody,” I remember being at a Cubs game when I was a kid and the looks we got when my cousin yelled “HEY, SODY MAN!” at the vendor.