Techum Shabbos does no apply in the city. It starts 70 Amos from the boundary of the city. Sounds simple? Not exactly. How are the boundaries determined?
Here are some links from OU I found. There are others but I have no idea who they are. I didnt access these so I can not tell you how helpful they are, but they for sure are going to be more accurate than what you can expect from the CR crowd
http://www.ou.org/torah/article/mitzvah_24_techum_shabbos
http://www.ou.org/community_services/video/77371