a little investigation is all you need to do
per castle creations
"For 6 cells NiMH:
Touring car (Associated TC4, Losi XXXS)
Short technical track - 4600 with good gearing range, or 5700 geared very small
Medium track - 5700 large gear range, 6700/6900 good gear range, 7700 geared small
Large track (nitros top out on the straight) 6700/6900 geared high, 7700 good gear range"
per novak a 7.5 [=to a 5700 mamba]
your gearings should be around a "7"
with a 88t spur
you need a 30 or 31t pinion
with 48 pitch you will need to use a smaller spur 
and a 24 or 25 pinion
all this information is available on the net