i think u are getting mixed up with the 43t bevelled gear in the front and rear diffs and the cen 47t spur gear that fits on the cen diff i used as a centre diff

not sure how to completely explain it without confusing meself!... the centre diff is part of the spur, if u like, as the spur is screwed to the diff casing the diff is driven "locked" with all wheels having equal traction, thereby giving power to the 13t gears u mentioned and onto the 43s etc

basically its no different to having a 46t spur gear on a single speed setup... cos thats all it is, but screwed to a diff