B4 Gearing

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Hi Guys

Looking for a little help with th egearing on my B4

My set up is as follows:

Mamba Max Pro
GM 97225 3.5T Motor
2s 5000Mah Lipo

What size pinion would be a good starting point with an 81T spur ? (or what sort of FDR should i be looking at as a starting point)
Will mainly be running on tarmac and short grass ?

Not looking for anything insane just nice steady bashing and long run times

Any help much appreciated

Thnanks Guys