This map comes from “The Face of Detroit: A Spatial Synthesis” in 1972 by Robert Sinclair at Wayne State University, Department of Geography.
“The Detroit automotive realm is the spatial expression of a vast midwestern manufacturing complex centered upon and organized around metropolitan Detroit. Though most often defined in terms of automobile employment or automobile facilities, this complex comprises an array of associated industries whose separation from the automobile industry is largely a matter of classification convenience. The rubber plants of Akron, Ohio, and Kitchner, Ontario; the metal foundries of Chicago and Cleveland; the machine tool shops of Cincinnati, Ohio, and Windsor, Ontario; the steel industries of Gary, Indiana, and Buffalo, New York; and the electronics and hydraulic research laboratories of Columbus and Dayton, Ohio, are almost as integral a part of the automotive realm as the automobile plants themselves.” (pg. 28)