SX2 Gearing help needed

[TS 4Whl]

After being away from racing (RC in general really) for about 6 or 7 years, I am getting ready to build a new SX2. I have been looking over the build options and going over some set-up sheets, but am still lost on a couple of points.

I will be racing on Indoor Dirt Offroad (large jumps, rollers etc.) track with a Clay or Clay base with some loose dirt covering it, type of surface. Longest Straight is roughly 100 feet or so.

Firstly Internal Ratio:
I am planning to start with a Brushed motor likely a 10 trun double or 12 turn double. Should I stick with the stock internal ratio, or what internal ratio should I be using?

Secondly Pinion Spur Combination:
What would be appropriate for a 10 turn double or 12 turn Double brushed motor.

Thanks,

Trevor