OD ratio 4.09 Laycock

I’m sure this question has been answered somewhere but I can’t find it; What is the overdrive ratio for the Laycock transmission (what would be the final drive ratio in overdrive)?

I think it will depend on which O/d unit you have, I there are differences;

The ‘A’ type was available with gear ratios of either 0.778 or 0.820 to 1 and the ‘D’ type with ratios of either 0.756 or 0.802 to 1.
I think Jaguar used the A type with the .778 ratio.
1/.778 = 1.285
So if the engine turns 1000 rpm in top gear the driveshaft is turning 1285 rpm.
With a 4.09 axle the wheels are turning 314 rpm.

Compared with 1000 rpm into my non-OD box in top gear and 3.64 axle, the wheels are turning 275 rpm.

None of them fit in an XK120. The chassis cross member would have to be chopped up quite a bit with a smoke wrench. :anguished:

Thanks guys; I was referring to the XK 140, (4.09 axle).


If you are referring to the original Overdrive ratio for the XK 140 then the table below might help you. I made this many years ago (but some things never change, especially those of the past :wink:).

So you see in the table that there were actually two different OD’s used for the XK 140 but both are of the 28 type meaning that (according Laycock de Normanville) the engine rev’s will be reduced by 28.5% with the OD engaged (just as Rob mentioned above).
Other available overdrives of the A type (manufactured from 1947/48 until the late 1960’s) had a different ratio as can be seen in this survey which also includes the 28 type as a reference:
Type 32 32.4%
Type 28 28.5%
Type 22 21.89%

Hope this answers your question.

Bob K.

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Thanks to all, I got it now.