unfortunately with the rustler their is not much in the upgrade area. like an alum. diff. gear or anything that is made with higher grade plastic that the gears are made of to beef it up. its alot of strain on that tranny in that truck. for what a gtb brushless system would cost just another 60 bucks will put you into a t4 or losi rtr truck that could handle such motors and give alot of fun and have better final drive ratio's than the rustler to get the speed he wants. i don't know what the final ratio is in a rustler but to get it to 40 or 45 i think one battery pack would destroy the entire tranny. to see if there is upgrade gears look at robinson's website to see if they make them for rustler.