plain and simple, the lower gearing (numerically) on the rear, the more the gears are designed for the highway, to keep rpm low and save fuel once you're up to speed... you will loose a little bit of economy going to a lower (numerical) gear ratio, while you're driving around town and stopping at lights constantly. higher gear ratio numbers will give you better low end torque, better fuel economy around town, but raise you're rpm on the highway, and decrease fuel economy.
for best selection of gear ratios, you need to weigh how much you drive in town/off road/need torque, vs highway speed driving
also, on 2wd vehicles, (maybe 4wd also, don't know) you can get an aftermarket, external over drive, which will lower your highway rpm with the touch of a button, though from what i know, they are very expensive and i believe require driveshaft shortening