E=MC2 happens to be Einstein's formula!
In the formula you reference, you aren't getting the distance between two
zipcodes; you're getting the distance between two points on the earth based on
certain assumptions. The two points being the centre of the region defined by a
zipcode and again assumptions are made regarding what constitutes a centre.
Trying to get the distance between two zipcodes as you asked is meaningless
because zipcodes do not define a a specific point.
Steve
PC Datasheet