Maybe I'm missing something, but if I type your 2 zip codes into google maps
60439 and 46514, it tells me these are about
118 mi – about 2 hours 9 mins
Elkhart, IN 46514
Please let us know if your 633.572201 miles is for this drive.
I think your number is about 5 times larger than it should be. But, maybe I've misunderstood part of your post.
Also, zip codes are "area locators" not points. Unless you have geographic centres of zip codes, your numbers will be approximate. So your 6 places of decimal are a little much.
I still think your distance calculation is too high.
You can test your numbers by putting 2 zip codes into google maps. See if it makes sense.
I have done distance calculations using formula in Access.