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