     Gears.exe computes the progression of your bicycles gearing.  This progression represents the number of inches your bicycle will travel under the particular gear configuration for each revolution of the pedals (assumes no coasting).  This progression number can be used as a standard across different bicycles to equate different gear configurations and wheel sizes. For example, two bikes geared at the same progression and pedaled at the same cadence (rpm) will travel at the same speed despite the fact that one has a 27 1/8" wheel and the other a 24" wheel.
     This is handy when touring in a group as all riders can adjust their gears to approx. the same progression.  It is also useful to map out the gearing in order to make a smooth progression through the gears to top speed.  This will often involve double shifting and to do this effectively you must know the rank order of your gear configurations.  Hopefully, the progression chart will show a nice spread of numbers rather than duplication of the same number across chainrings.  If duplication is rampant, perhaps a change of one or more of the rear cog gears is warranted.   
     To use gears.exe you must have the following information: the number of gears on the chainring,  the number of teeth on each of these gears, the number of gears on the rear cog,  the number of teeth on each of these gears, and the size of the rear wheel.  In addition, if you wish to compute speed you must provide the cadence at which the bike will be pedaled.  
