I'm definitely a midwesterner, born in MO, lived all over the place, and now live in MN.
I've called it "soda" my whole life. "Coke" was definitely confusing in the southeast. "I'll take a coke." "What kind?" "Wtf?"
I think the real lexicological definition is "ope"
Do you ope? If so, you are a midwesterner, there are various flavors of midwesterner, but that is one important characteristic.