S series is 3.9
can't adjust a cable, you can change the drive gear in the box to suit. There's a few combinations you can achieve with the gears.
Tacho comes from engine, has nothing to do with diff.
Random diff info I found out the other day. YR30 Tarago F-series axle tube flanges are bigger than T-series size which I believe is the same as E-series. Will confirm in a few days when I try to put corona disc brakes on the E-series.
MX13 F-series axle tube flange are the same as T-series.