1)   Off The Road Metric Tire Size Calculator 1
As any 4x4 or car enthusiast should know, when you are dealing with formulas dealing with gear ratios and such - you will need to know the tire diameter in inches; this calculator will offer just that.

 2)   4x4Calc 1.00
4x4OffRoads.com - 4x4 Off Road Calculator Use this program for calculations for your modifications project. It currently has three mini-calculators. Select your calculator by clicking on the various tabs.