What pinion???

[Hyper-1983]

Hi there
Currently race a b4.1 ft on a high grip astro indoor track! Esc is a Novak gtb and the motor a Novak 10.5 ss! Using the kit spur! Just wondering what pinion size I should be using! Currently using a 25 tooth and struggling to keep up with these new stock spec esc equipped cars! Will be getting new electrics in the not so distant future but just want to check that my gearing sounds about right!

Many thanks.