The Remote Control (RC) truck gearing ratio information is available here:
https://sites.google.com/site/rcgearing/
Listen for noise: if you have it set correctly you should not hear any noise coming from the gears. It is too loose it will cause gears to wear also, or to slip and clash and ruin the mesh.
Adjust the pinion to the spur with the paper between them, and then tighten the motor mounting screws, and you should be able to pull/work the paper out without it tearing it off.