XRAY - Model racing cars

I always be able to figure the FDR for 2wd, however for xb4 4wd with 1:2.5 ratio how do I do the formula ?

the decimals are different ? (:) (.)

AE SC5M 2.60:1 ratio Spur 84/19
84/ 19= 4.42 x 2.60 = 11.5 FDR for 6.5 motor

XB4 81/20=4.05 x 1.2:5 ??? getting 4.86 fdr

The internal ratio of the XB4 is 2.5 (35/14) , so the FDR in your example (81 spur / 20 pinion) is 10.125.