People often use Detroit to refer to the North American automobile industry in general, and particularly the Big Three auto manufacturers (General Motors, Ford and Chrysler), in a similar manner to the usage of Hollywood to refer to the movie industry, Washington to refer to the United States Government, Nashville to mean the country music industry, and countless other examples.
Since the epicenter of the motor industry on the North American continent is indeed Detroit, Michigan, this makes sense, but the term is used to refer to American automobiles in general, regardless of whether they really come from Detroit (and there was a time when there were numerous other American manufacturers, not all of whom were headquartered in Detroit).
Used in such expressions as 'a big hunk of Detroit iron", "Cars coming out of Detroit that year were smaller than in previous years", and so forth.
| 
