i would like to use Gmap API to calculate the best driving route for many destinations. I have read this page:
http://googlegeodevelopers.blogspot.de/2010/03/good-day-f
You might want to look at the Distance Matrix service: https://developers.google.com/maps/documentation/javascript/distancematrix
It gives you distances between a set of origins and destinations, and might help you with narrowing down options.