Running the numbers through BrianG's speed calculator:
http://scriptasylum.com/rc_speed/_top_speed.html
A 13t pinion should be perfect assuming a 46t spur, achieving ~45mph top speed.
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz