what pinion?

im noob to gearing either though ive been into the hobby for years and years... just 1 quick question... i got a e-revo with hobbywing 80A esc and a 9xl motor im going to use 4 cell A123 battery.... what pinion should i use ? i got a 17 tooth one on the way but is that even close to what i should use? sorry if this sounds like a (not again a darn noob question) haha.. i promise i wont be bugging much more than this... my main use is bashing around with my kids so speed is not the most important thing mainly backyard bashing and some street runs...
Pete