There are many different gear design calculations required when building a gear set. It will depend on what the gears purpose is as to which ratios are the most important. Some gears need to be very strong, others have a main concern of distance or size. Diameter of the gears, number of teeth and ratio of one gear to another will all come into play. A great site to see the different aspects
. of gears is http://www.roymech.co.uk/Useful_Tables/Drive/Gears.html.