Building a SUPER MONSTER Crawler

[jaxxon3007]

I want to build monster truck, using crawler parts , i loved the idea of motors on the axle , so i went ahead and bought Integy Type I Professional Crawler Chassis, and all the parts that i though would suit me
NOW, for the axle i went and bought Metal Gear Rock Crawler Alloy Front/rear Axle w/ Spool for SS01 (crazy stuff) . Im looking to put on some 4068 Sensorless Brushless Motor 2650KV f 1/8 scale , but i dont know what pinion gears i should get , pars are expensive and i dont want to find out by wasting my money. If someone can point me a link with what pinion gear i should get for that kind of motor , i would be happy so i can finish this project Thanks