King Cab / Thorp Ball Diff FDR

[Lhodgo 208]

Is there a “go to place” to obtain the internal transmission gear ratio for Tamiya? I’ve got a thorp diff and it’s got a 78 spur gear. The kit spur was 77. But I need to know the transmission gear ratio to work out what pinion I need to get a 9.0 FDR??? Need my tamiya buddies here...

Diff gear 31
Counter gear 21 
Input get on spur 16

Spur 78
Pinion ?
FDR 9.0

[Lhodgo 208]

Also just looking at the spur it seems to be .48 pinion...prob makes sense because it was US? Would a 0.6 mod kill it?

[Kaminagayuchi 41]

48DP and 0.6 are different, it’s impossible to work together. 

For me, but I ´m not a specialist, Thorp make 48 and 32P.