This is an upgrade kit for the AM14U4 that includes 2 EVO Slim gearboxes for CIM motors (not included) and the necessary hardware to mount them. The gear ratios for this kit have been optimized around a 6 in. wheel and some middle of the road driving speeds. For the 6 in. wheel version we give you the 10.86:1 ratio for a final drive speed of ~11 ft/sec