Much depends on which motor and ESC you have and whether or not you are using timing advance or turbo boost but if it helps you at all, I've been running an Xcelorin 6.5T with a Nosram ITSC v2 all year with no turbo or advance and I've been geared on 21/76.
I understand those that run the SP v3 motors tend to gear higher but you can always start off conservatively and gear up if need be.
HTH
|