I always thought of Metropolis as New York and Gotham as Chicago, which would make it about 800 miles.
But I know that NYC does exist in the L&C universe (it was mentioned in Man of Steel Bars) and I think Chicago may have been mentioned at one point as well, in which case - heck if I know!
Lisa, who gets a little dizzy at the fact that in some universes, Metropolis is apparently in Kansas.
