Tire Gear calculator sites:

The Mother Of All Tire Guides: http://www.d-p-g.com/jp/fall96/wjpfa96.htm
Randy's Ring & Pinion Service: http://www.ring-pinion.com/
4LO.COM: Tire/Gear Calculator: http://www.4lo.com/
Speedometer Gear Applet: http://www.jeeptech.com/~txh3202/

john's gear formula:   http://www.wagoneers.com/john/JEEP/GEARS/gears.html