You can use the Speed Calculator made by BrianG to help with gearing:
http://scriptasylum.com/rc_speed/_top_speed.html
Generally it is best to gear 1/8 for no more than about 45mph to keep temps and controllability in check.
For you:
-1/8 buggy typical ratio (43/13)
-1:1 transmission ratio (direct drive)
-53t spur
-5s Lipo @ 3.6v/cell (I use this value to account for voltage drop)
-6in. tall tires (this is what I use to account for ballooning)
Plugging those values in, a 14t pinion would yield 55mph+, which IMO is too much. An 11t pinion would be optimal.