The spare change that people in the city ask for. One is likely to pay the tax near places visited by suburbanites (e.g. Greektown, Comerica Park, Wayne State University campus) or while stopped at traffic lights on expressway service drives.
Mom: "Here's five bucks. Can you pick up some bread for me at Avalon on your way home?"
Kid: "Avalon? Then, I'll need an extra fifty cents to pay the Detroit city tax."