Gearing a Cat SX2
 

I use a 9t motor, 2s 5000mah lipos, Im not sure what spur or pinion to use? I want some good speed and acceleration, but the motor and electrics not to get hot, what should I gear it as?

It will depend on what speedo and motor you are using and also if your speedo has timing advance software.
I would estimate around 83 spur (std) and 23 - 24t pinion, run a couple of mins then take temps.
Again safe temps will depend on what motor you have.

Thanks.