Hey I have found a cheap set of different size pinion gears and I was wondering if they would all fit the rustler Here they are
http://www.ebay.com.au/itm/190751998...#ht_1484wt_813

Here is the information they give you (plus an image)
Robinson Racing
48 Pitch Odd Pinion Gear Set

The SIX-PACK Starter Set is an economical way to test different gear ratios before buying lightened and hardened premium pinions. There are 6 different precision cut pinions that let you find the optimum ratio for you and your machine. 1/8 inch bore. Six 5-40 set screws included. The odd set includes 15, 17, 19, 21, 23 and 25 tooth pinion gears.

Part Number: 1055