Using BrianG's speed calculator: http://scriptasylum.com/rc_speed.html
I figured that using 6" tall tires and 1.10v/cell in 2nd gear, a 22t pinion would be good to keep ~35mph as the upper limit.
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