America now has only one tank factory at Lima Ohio. They haven't build a new tank from scratch at Lima since the mid-1990's as all tanks are reconditioned, but they are reconditioned to such a degree that they are practically new tanks.
M1 tank reconditioning at Lima averages half a tank per day (15 tanks a month). General Dynamics has stated that it can easily ramp that up to two and a half tanks a day (75 tanks a month). In wartime that figure could conceivably rise to over a 100 tanks a month. If we say that reconditioning takes the same amount of time as producing a new tank then that would be up to 1,200 tanks a year. Building another tank factory or re-commissioning the still existent Detroit Arsenal Tank Plant would not be that hard but it would probably take at least six months to either build from scratch or refit with the right machine tools and equipment. So with the right infrastructure it is possible that America could build up to 2,400 tanks a year after six months or so.
Some discussion on this here...
http://forum.juhlin.com/showthread.php?t=4627