Short description of Off The Road Metric Tire Size Calculator 1: OTR Trading made yet another handy application for all the 4x4 extreme fanatics out there. 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 for you and allow you to quickly work out the inches diameter from your metric size of your tires for your 4x4 or your normal road car.