im not sure but i can say that the xray 808 had large drive gears (clutch bells) and smaller spurs...like 16 or 17T with a 41 or 42 if i remember correctly....arguably this was not ideal in terms of what the pro guys required i guess....so with the xb9..their latest buggy..they went back to a more conventional gearing found on alot of newer cars..ie..a 13T bell and a larger spur gear...im not 100% sure what the sixe on the xb9 spur is but for on the mugens most guys run 44T to 48T. i guess like all things it comes down to preference, engines and track conditions probably