This program is designed to help you (the user) find the appropriate gear to use so that your speedometer reads correctly.  The gear you'll need to buy is called the 'Driven Gear' and is located on the end of your speedo cable.  To find out how many tooth Driven Gear you need to buy, first you need to know how many teeth are on the drive gear which is located inside your transmission on the output shaft.
 The Ford SVO catalog lists these:
 6 teeth (Black) - for some 4-cyl T5 transmissions.
 7 teeth (Yellow) - for most '83-'89 V8 5.0 Mustang transmissions.
 8 teeth - for '90-'95 Mustang GT and 5.0L LX transmissions.
 To be 100% accurate check yours!
Drive Gear Teeth: Six   Seven   Eight Tire Size:  /   - 
Manual XR4Ti (3.64)    Auto XR4Ti (3.36)    Custom Ratio: :1
Driven Gear = Teeth
Ford only has available up to 21 teeth, there are aftermarket companies that sell 21+ tooth gears but at these amounts the teeth are next to paper this and you would be better off installing a drive gear with less teeth and then re-calculate.
Note: This formula used for this calculation was taken from the Ford SVO catalog, according to Brad Anesi that formula is off by -1.  In this version I am adding +1 to the end of the equasion found in the Ford SVO catalog