If you have something that gives latitude and longitude of each zip code reference you could run some sort of vector evaluation between two known points. Without that, I would say you are out of luck as there is no particular direct relationship between zips. The first three digits refer to the regional distribution facility, so the odds are that if the first three digits are different in your comparison you are likely more than 15 miles apart, but not necessarily. For more rural areas, the actual sequence of zips seem to be alphabetical within the regional distribution area so even if the two zips are only one digit off, that does not mean they are adjacent. Similarly a major metropolitan area may have one set of zips (including the first three digits) that vary from the three digit zip for the surrounding rural area, even those some of the localities within the regional area may be within 15 miles of the closest of the rural areas adjacent.
If you are interested in a comparison between any portion of a zip code area to another area you are really out of luck because in general there is not a clean (straight) boundary between area.
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