28” and larger tires (can also be used with some of the heavy 27” mud tires). A lot of power can be lost from adding big tires. This kit is for re-calibration of the rate of shift for 28” and larger tires. Improved acceleration and response with extra large tires. Includes a new primary spring, custom helix, and a new secondary clutch spring. The instructions have a set up guide that explains how to set up for tire sizes. Using this kit with smaller than specified tires can hurt top speed.