CB...use the kit supplied 900 wt in the rear. you will be very pleased. most racers at my local track run 1000wt the only guys running thicker are the mod guys and they run btwn 1500 and 2000. I run 1000 in my spec r diff on the rear of my ta05v2,17.5 rcgt class and I place mostly 1st or second (not to brag). i did not notice a difference btwn 1000 and 1500. at 1000 the car drives well and i had to go up a rate of springs all the way around to limit the chassis roll when i switched to the gear diff so the wheel would not lift on sharp corners.white hard ft and rear. we run on a high traction outdoor track(see village raceway thread in florida fourms) the tao6 has less roll which is why I am assuming tamiya used the 900wt. Stay under 1500 and you will be a happy racer
you can see my car and setup in the v2 fourms.