revo pinion

I am converting my revo 3.3 to brushless and want to know what pinion to use, i am a newbie to electric, but not to rc's. I have ordered a feigao 7xl, and i am going to use a mmm v3, and zipp-h lipos ( 5000mah, 5s, 20-30c ). I will lock second gear, and the spur is a 38t. what pinion should i use if im going to race at a medium size track, and want to bash with some buddies. I can order a couple if i need.