Not sure on the internal ratio of the RB5, but if it like the B4/X6, I would say 26/27 should be about right - depending on track layout (tight/twisty then 26, more open then 27). I ran 26/81 on my 7.5 in the X6 around both Broxtowe and Grantham and they both seemed fine.
From what I have been led to believe, the SP motors need a larger pinion than the equivalent LRP...
HTH
