gearchart.com is useful, mute your audio before using it though.
Excel is a powerful tool for figuring gearing.
For what you have listed, tc5/mamba 5700/assuming rubber tires, I would start at 6.5 FDR.
FDR = spur/pinion * transmission ratio.
Be prepared to go higher/lower with pinions based on heat/speed needs.
-Dangerous with power tools-