Speedos in XK140 overdrive cars

I have a question for the owners of XK140s originally fitted with overdrive gearboxes. Can you kindly check your speedometers for me? I am looking for the numbers on the face of the speedo, something like X.51691/35 with the number 1200 also.
The issue is that the usual parts book lists the /35 speedo as being fitted to standard 3.54 rear axle ratio cars, and does not list any speedo number for the 4.09 rear axle cars (i.e. the rear axle ratio fitted to OD cars).
However, the supplementary book for the DN overdrive lists the same /35 speedo for overdrive cars, which seems to be an anomaly.
Further, for six XK140s sold new in Sydney, the factory car cards recorded that same speedo number /35 for four cars but /34 and /21 for two other cars. In addition, S814982DN and A819020DN both have /35 speedos.
Roger Payne told me that one of his reference books nominated /53 for the 4.09 speedo.
Back in 1998, on the old email list, Rob Reilly speculated that perhaps the speedo drive on the OD unit had been set up so as to replicate the revolutions suitable for the speedo for the 3.54 rear axle – but Viart/Payne in “Explored” say that Smiths in fact made a speedo suitable for the 4.09 rear end and that this was either X.51691/44 (875 rev/km) and /43 (1375 revs/mile). No source for that information is stated.
Confused? Of course! But I would love to accumulate a few more examples of the speedo numbers on XK140 OD cars and see if any particular pattern emerges.
Regards to all, John Elmgreen

John … I have a 1956 XK 140 MC, FHC with overdrive and the number on the speedo is X. 51691/35 with 1200 marked under this number.


My 1955 XK 140 FHC with OD had the same version as Harv’s: speedometer X51691/35. It was later modified as I had opted for a lower axle ratio (3.72 instead of 4.09)
As far as I know, the OD gearboxes are the only ones that use Angle Drive C.8060. (Smiths X.80630) later replaced by Jaguar C.24589 (Smiths BG2402/07). Is there a certain reason for the use of that Angle Drive (other than space issues for the cable)?
There are references that these Angle Drives had a 1:1 ratio which would not explain how a 3.54 Speedometer was placed on a 4.09 final drive car.
Other Angle Drives like Jaguar C.17087 (Smiths BG 2402/02) had a 1.27:1 ratio (14/11 gears) so it is not uncommon to have angle drives with other than 1:1 ratios.
Rob Reilly mentioned already in 1998: Apparently the right angle drive has its own reduction which cancels out the 4.09/3.54 difference?? This equals about 86.5% or a reduction of 13.5%. I cannot remember whether I’ve ever tested the ratio of my Angle Drive to see whether it was really 1:1.
Another possibility is that the speedometer connection to the Gearbox has a different (internal) ratio compared with the speedo cable connection on the Overdrive. Remember that the speedometer connection had been developed and was manufactured by Laycock and not by Jaguar and this overdrive was also used for other brands. Gear wheel 22 (see below) may have had a different ratio.

Bob K.

Oh, no! Something I wrote 22 years ago is being quoted? I hope it was right. :cold_sweat:

As I happen to have a JLE OD box apart here, I can add a little information to this discussion.
The speedometer drive gear is machined into the main shaft, not a separate ring as on the JH and JL gearboxes. It has six teeth.
The driven shaft has 15 teeth.
So for every rotation of the main propeller shaft, the driven shaft turns 6/15 or 0.4 turns.
The drive exits the box up and to the left, where on the JL it is down and to the right. Therefore the right angle drive was needed to aim the cable where it was wanted.
I have a Smiths X80630 but it is broken inside and won’t turn, so I can’t get the ratio.

John i have S814191DN with speedo no’s .X51691/35 1200

@John_Elmgreen Smiths X.51691/43 ( 1375 TPM ) speedometer was fitted with 4.09 axle WITHOUT overdrive.

If you do the maths to calculate the Turns per Mile (TPM), using constant speedometer gearbox take-off ratio and tyre size, for the 3.54, 3.77 & 4.09 Salisbury axle ratios the results are:

3.54 - 1200 - X.51691/35
3.77 - 1280 - X.51691/41
4.09 - 1375 - X.51691/43

The X.51691/35 1200 TPM speedometer is shared by 3.54 & 4.09 OD because the speedometer gearbox take-off ratios are different i.e. a different speedometer driven gear and/or the angle drive. The speedometer driven gear (C.843) fitted to a non overdrive XK JL/SL/JH gearbox has 11 teeth (see @Rob_Reilly later post), so different from the JLE gear he gave details and posted a photo of.

Interesting you mention one of the new Sydney XK140s had Smiths X.51691/34 speedometer fitted; this is not listed in the SPC. Do you know the axle ratio of this car? Could it be a bespoke order or a typo? Speedometer X.51691/21 is ENV 3.64 but also 1200 TPM, so the same internal calibration as X.51691/35


S818919DN has X.51691/35 with 1200 on its speedo.

Looking at the right angle drive, it looked like there are Welch plugs holding it together. I figured it’s broken anyway so what have I got to lose. So I took it apart. Tapped each shaft on the small end with a punch and the Welch plug popped out.

There are 10 teeth on each gear, so the ratio is 1/1.
There is supposed to be a swiveling square end on the shaft with the notch, which would fit into the driven shaft inside the gearbox, but it is broken off and lost.
And the reason it wouldn’t turn is it was full of rust and crud.


I guess (thanks to the collective work of many) the conclusion should be that the XK 140 speedometer drive on the Laycock Overdrive has a different ratio (6/15) compared to the one the (non-OD) Jaguar/Moss gearbox (6/16??).
Because the Angle Drive C.8060 has a 1:1 ratio, it plays no role in the question why a 3.54 axle ratio speedometer can be used for a 4.09 rear axle on the OD version.
The difference in rear axle ratios.(3.54 versus 4.09) of 0.86 is (almost) compensated by the difference in speedometer gears on the gearbox versus overdrive (6/15 versus 5/15) of 1.2, leading to a small misreading of about 3% on the speedometer.
I learned a lot from your question…

Bob K.

Here is a set from an SH gearbox, the same parts are used in DH, JH, SL, OSL and JL non-OD boxes.
C.910 on the mainshaft has 5 teeth and C.843 the speedometer driven gear has 11 teeth, so as the prop shaft turns once the speedometer takeoff drive turns 5/11 or 0.4545 turns.

@Rob_Reilly @Bob_K1 Apologies, I mistakenly identified my speedometer gear drive with 16 teeth as C.843. The real C.843 has 11 teeth, with convex machining below the teeth that matches the profile seen in the SPC gearbox diagrams.
So as not to confuse the thread I’ve updated my earlier post.

And thanks for that confirmation. I wondered about that when I saw it.

Rob, Could we see a photo of your JLE OD speedometer drive gear? Does it also have convex machining?
Unlike the XK140 the XK150 SPC includes the Laycock JLS overdrive unit but in the diagrams the drive looks different; the teeth are near the middle of the shaft and it has a stepped nose. Part C.11953.

So we give it another try to see whether the numbers match.
Non-OD gearbox has speedo ratio of 5/11 or 0.45
Laycock OD unit has speedo ratio of 6/15 or 0.40
The difference between the two speedo cable ratios is therefore 0.40/0.45 or 0.888.

Speedometer X.51691/35 is used for both the 3.54 diff as well for the OD cars diff with 4.09. The difference between the two axle ratios is 0.865.
There is now only a difference of 2.6% in the speedometer reading which seems very acceptable. I hope everybody can agree with this explanation why Jaguar used the same speedometer for the standard 3,54 and the OD 4.09 axles. Conclusion: Jaguar was correct…

Bob K.


I have been following this and ran the same numbers. I concur. My interest was that I have an OD fitted into my XK120 with the standard diff. By my reckoning my speedo should be out by about 12%. Proof will be when I get it back on the road next year some time.

I have my JLN OD apart at the moment as well as an early standard JH gearbox and all the parts are identical to Rob’s photos.

Interesting discussion.



An OD in the XK 120? I understand it requires quite some “surgery” of the chassis to make enough space for the longer gearbox + overdrive. Did you manage to get it all in there?

Bob K.

It’s a straight gear.


The chassis was modified, change was done many, many moons ago. I posted a photo of the chassis change in a post earlier regarding early gearbox covers. This was no hack job as some have been, rear mount is the original slab type with a large shackle over the end of the OD. I wouldn’t bother with the work myself but am happy to leave it that way.


Many thanks Rob,

It looks just like the driven gear drawn in the XK150 parts diagram (C.11953), which also has a sleeve with a threaded end to fix the angle drive. The TPM values printed on speedometer dials for XK140 and XK150’s fitted with OD are both 1200, so presumably the same 15 teeth driven gear (& 6 teeth annulus assembly) was used for both JLE & JLS units.

Does your driven gear have the sleeve?

I have an original overdrive car with an original 4:27 rear end. The speedo is a 51691 / 41. I don’t think anyone has mentioned one of those in this thread. I can’t tell you whether it is accurate as I have never been able to drive the car. Getting closer though.