Rick Aschmann has made a hobby out of studying and mapping North American English dialects:

This is just a little hobby of mine, that I thought might be interesting to a lot of people. Some people collect stamps. Others collect coins. I collect dialects.

Aschmann goes on to explain the map:

There are 8 major English dialect areas in North America, listed below the map at left. These are shown in blue, each with its number, on the map and in the Dialect Description Chart below, and are also outlined with blue lines on the map. The first 6 of these begin at the eastern seaboard and proceed west, reflecting western settlement patterns.

There are even several hundred YouTube links clickable through the map that serve as subdialect samples. What dialect do you speak?

[Aschmann]