An example:

I have two fixed gear mtb / tourers that are almost identical as far as wheels and tyres go and one runs a 48 tooth ring with a 17/19 in the rear for a 62.8 / 70.2 gearing and the other runs a 42 with a 15/17 combo that has a 61.5 / 69.6 gearing.

This is a perfect gearing for bikes that tend to be loaded up with gear and get ridden longer distances...the slightly lower geared bike tends to carry more (it has front and rear paniers), has slicker higher psi tyres, and gets taken on many longer rides.

One could use the physical method of measuring gear inches but for practical purposes, Sheldon Brown's Gear inch calculator is a perfect tool to help you figure things out... an inch one way or the other isn't a big deal.

http://www.sheldonbrown.com/gears/