I have a question on DF03 gear ratio

I want to put a 12T brushless motor on my DF03.
Which spur gear and motor gear should I use? (how many pitch?)

05 pitch

Oic!
Actually,  I dun really know how does that work.
Could any brother tell me about how to select the most appropriate gear ration??

I used,
78T spur gear with 29T motor gear with 9.5R motor = very very fast and motor very hot. Difficult to control.
after that I changed the motor gear to 27T, still very fast but motor temp ok, control much better.
Now using the 26T motor gear, very good for on-road buggy.

Gear Ratio     
               Spur
Pinion      78T        82T        85T
23T                     10.89     11.29
25T                     10.02     10.39
26T         9.17       9.64
27T         8.83       9.28
29T         8.22