The gearing - gearbox and differential

Overall gearing

The overall gearing on any Oxford, Cowley or Isis can be played with. On any of these vehicles 1st gear is not really needed; indeed I usually start in 2nd gear and when cruising along in the country, I quickly feel that 4th gear is not high enough!
Fortunately for us, we can mix and match bits from other BMC cars to remedy these complaints.

The Gearbox

There's not too much I can say here, except that both 4-cylinder and 6-cylinder gearboxes were available on later/more sporting models with closer ratios (ie. a close ratio 'box). However, these innards will be harder to find, more expensive, and because the Morris' had column change levers, you can't fit a floor change box unless you start cutting holes in the floor!

Later series III Oxfords and Cowley 1500s had the option of a central floor change gearlever. This gearbox or a later 1622 floor-change unit can be fitted into a column change, however you'll still have to cut a hole in the floor, and anyway a centre floor-change gearlever is a very mundane thing; I mean - all modern cars have them, you know ;-)

I must also mention that the later all synchro MGB gearbox (and overdrive) won't easily fit - they're too big to fit into an Oxford/Cowley.

Overdrives and Differentials

Cowley and Oxford

Go to Isis info

The overall gearing on the Cowley and Oxford is fairly low:

Oxford is 15mph per 1000rpm, giving a frenzied 4000rpm at 60mph!
Cowley is 13.8mph per 1000rpm

The contemporary Austin A55 offered an overdrive option (Borg-Warner), and there is the MGB type overdrive too (Laycock). However, to fit any overdrive would require some modifications to the floor pan and more, otherwise the Oxford would've had one. I have no experience or information on this yet, except that the A55 overdrive unit is the same as on the bigger C-series cars (Isis, A90 etc.) and on the A55 required the Cowley's lower diff ratio. You will have trouble finding an A55 overdrive 'box - they're very rare!
Given that first gear is so very low, (and the fact you won't find an A55 overdrive, or make it fit easily) it's not too silly to think about the possibility of higher overall gearing with a higher ratio differential, especially if you're going to uprate the engine.

The differentials fitted to the Oxford and Cowley are of the B-series type.
Here is a brief list of which ratios were used, and where:

Ratio	Vehicle it comes from	Overall Ratio
5.125:1	Standard Cowley 1200, Oxford 'special'	13.8mph/1000rpm on 15" rims
4.875:1	Standard Oxford, Austin A50	15mph/1000rpm on 15" rims
4.55:1	MG Magnette ZB, Farina Oxford V et al.	c.16mph/1000rpm on 15" rims 15.6mph/1000rpm on 14" rims
4.3:1	A55 mk1, Farina Saloons 1963 onwards	16.6mph/1000rpm 14" rims ?/1000rpm on 15" rims - I may find out soon :-)
4.1:1	Some models of MGA	No idea.

Isis

The Isis is fairly well off for overall gearing (c.19mph/1000rpm), but first and fourth gears could still be considered too low.

The Isis did have a factory option of an overdrive gearbox, and it's a great thing to get hold of. It adds another, very high ratio giving a lazy 27mph/1000rpm.
The overdrive is a Borg-Warner semi-automatic. The same overdrive gearbox and controls can be found in the Austin A90, A95, Wolseley 6/90, Riley 2.6.
However, it's not just a case of bolting this overdrive unit to the back of your gearbox. It needs a longer mainshaft, and electrical kickdown switches etc. fitted, so it's a bit fiddly.
Just like the Oxford and Cowley, we can take a higher ratio differential from a different BMC vehicle, which is a straight swap; less fiddly and cheaper than the overdrive option.

The differentials fitted to the Isis is of the C-series type.
Here is a brief list of which ratios were used, and where:

Ratio	Vehicle it comes from	Overall Ratio
4.1:1	Isis, Wolseley 6/90, Austin A90	c.19mph/1000rpm 15" rims
3.9:1	Austin A99-A110, Wolseley 6/99-6/110 with Manual Transmission	?
3.54:1	Austin A99-A110, Wolseley 6/99-6/110 with Automatic Transmission	?