You can buy zipcode data pretty cheap (around $250 for a liscense) that contains the Latitude and Longitude of the center of each US Zip. Most of the companies that sell this, provide the formulas to calculate distance between two zip codes using this info.

I used it for a project before but I don't have the name of the company handy. I'll look for it and post it if I find the company.

UPDATE
The zipcode db I was speaking of is available at zipinfo.com. They provide several ways of calculating distance with various degrees of precision.

-Lee

"To be civilized is to deny one's nature."

In reply to Re: Zipcode search engine by shotgunefx
in thread Zipcode search engine by Jerry

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.