Have you tried the speed calculator yet? If not, here is the link:
http://www.scriptasylum.com/rc_speed/_top_speed.html
Remember though, the XT8 has a 10t pinion in the diffs so it's 43/10 - not 43/13 like many others. And one other thing to keep in mind is how you plan on using it. Are you looking for a good racing setup? If so, what type of track will you be on - tight and short or long and wide open?