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Fitting an Overdrive Gearbox

Parts Needed

Overdrive gearbox
Overdrive Propshaft (shorter)
Overdrive Switch, Relay and Wiring loom
Overdrive Speedo Cable (longer)
Gearbox Oil 20/50 engine oil don't gear oil or synthetics.
Additional Parts
Speedo
Gear Lever if upgrading to all synchromesh (can use Hillman Hunter)

Overdrive Gearbox

The best gearbox is the series V as it is all synchromesh and has close ratios. All synchromesh gearboxes have reverse on the right.
Later Hunter type gearboxes can be fitted with a lot of trouble because of different spines etc., so are best avoided unless you need the stronger overdrive unit.

Overdrive Propshaft

You will need an overdrive propshaft or have yours shortened as the gearbox is longer, the overdrive propshaft is 29.5"or 74.9cm between UJ centers see picture.

Length between centres (of UJ's) Standard - 32.25" 81.9cm O/D - 29.5" 74.9cm
Overall length Standard - 39.34" O/D - 36.59"

Overdrive Switch, Relay and Wiring

This will be either a normal switch and relay (6RA 33213) if series I-II or self-cancelling relay (6RA 33199) if series III-V see below for wiring, I have simplified the Rootes wiring diagrams.

Speedo

The drive gears for the Speedo are different in the overdrive unit, plus Rootes often fitted a lower ratio back axle of 4.22:1 with an overdrive gearbox, instead of the standard ratio of 3.89:1. Rootes fitted different Speedo’s to overdrive cars to correct this, but unless you change to an overdrive back axle ratio (4.22:1) the Speedo will still read wrong, I would not recommend fitting the 4.22:1 overdrive ratio, as it is a bit low for modern roads but does give slightly better acceleration. I fitted an overdrive Speedo(will read 5% slow) then recalibrated by removing the Speedo needle and refitting it at 30mph (using SatNav for speed) it won't be correct at all speeds but will be at most speed trap speeds.

IV-V Speedo Marking

Speedo non-overdrive is marked 1020 TPM(turns per mile)
Speedo overdive is marked 1504 TPM(turns per mile)
Note larger or small size wheels and tyres(low profile) also affects TPM

Gear Lever

If changing from non-synchromesh on first gear to an all synchromesh gearbox you will the need a matching gear lever or cut yours off above the ball, turn 180 degrees and re-weld, as reverse is on the other side. Hillman Hunter gear lever can be used, the shaft is slightly thicker at top

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