E-maxx w/ Feigao 540C 8XL pinion selection

[refusalspam]

I'm planning on getting a Feigao 540C 8XL, BK Warrior 9920 for my E-maxx. I'm wondering what pinion gear size to use with this. I'm planning to start running with 12 cells, then upgrade to 14-18 cells or 5s-6s LiPo's in the future. I'd like to be able to hit 40mph w/ 12 cells. I currently have stock gearing (66T spur).

From my calculations it seems like a 20T pinion will match the same gear ratio as other members here, but when using the speed calculator http://www.scriptasylum.com/rc_speed/_top_speed.html the top speed goes to 33-35 mph. Should I stick with this pinion or go up a few teeth? I don't want to burn up my ESC.