OK, I am in the middle of a bandit project but have a dilemma...

I talked to one of the guys at traxxas and he sent me an e-revo spur and pinion but the 62t 39p is too large. I looked online and found a 55t and am wondering what the size difference is as far as outside diameter so I can guesstimate something for the pinion. it will definitely take several tries and at $9 a pop, you can guess it wont take too many to figure it out or just give up!!