I run the 21T 550 with a Hobbywing 1080. Currently running a 10T pinion and 68T Spur. Mine is a light weight crawler with a short WB. This combo is slow, but boy does it work on the rocks. Plenty of wheel speed and as long as you can get a tire on it, will climb anything. Right now it is my best comp rig. I was saving for brushless, but now plan on running it as is.