I want to use ArcMap 10.1 to calculate the straight (euclidean) distances from zip codes to their nearest city centers in about 300 U.S. metropolitan areas. I have two point shpfiles: one includes the centroids of all zip codes and the other includes the points that represent city centers. Both of the two shpfiles are in the GCS_North_American_1983, which prohibits me from doing the distance calculation.

I have converted the GCS into North America Lambert Conformal Conic to do the calculation.

It worked but what is the best way to do it?