Hey dave, welcome to Rc-monster

If your looking to race the 1512 series is the best beat, Probably a 3.5d or something around 1500kv to keep RPM down and the motor most efficient.
gearing is questionable, it depends on your track.. 35-40mph is the norm for racing.
http://scriptasylum.com/rc_speed/index.html
^^^ that is THE best tool to calculate speeds etc, kindly provided by one of our very own members(brianG)
All of the parameters needed are preset into it.
Edit- did some calculations, it looks like this
12t- 35mph
13t-37-38mph
14t- 40mph