Schumacher kf pinion gears

[jackh84]

I recently bought a used kf roller. Ive fitted my electrics that I took from an sv2. I have an 8.5t motor the csr has a 78t spur gear fitted atm. The pinion on my motor is 25t can ill be using the car at Kidderminster track and at Midlands raceway indoor track can anyone advise on the correct pinion gear for this kind of usage. Im fairly new to all this any advice would be appreciated.
Regards jack