does higher gearing increase rpm or just the voltage?

[suicideneil]

170 is the maximum for a feigao motor really, any taller gearing and it will likely get too hot and de-magnetize the rotor > toast.

Keeping the motor and esc temps is check is vital- at that end of scale, your motor is on the edge, one tooth higher on the pinion may be possible, may be bad news.

Going from a 68 to 58 spur is a huge leap- your motor will fry for sure.