A tire has to fit on the rims and in the fork/frame. Use the ERTRO number to determine the correct circumference (e.g., a 700c rim takes a xx-622 tire, with xx being the width in mm). Use the inside rim width or manufacturer's recommendations to determine acceptable tire width for your rim. Measure the fork and frame to determine whether there is adequate clearance without rubbing (e.g., 29" tire has an approximate overall diameter of 29"). Sheldon Brown is a good source for additional information. http://sheldonbrown.com/tires.html