Speedo drive gear question

(sawyer10702) #1

On my series one I replaced transmission. To original now the gauge is of by about 8 mph. So if my Speedo says I’m going 75 I’m really doing 8mph less. Are there more teeth on the gear drive that goes into its home. Any thoughts any person’s might care to explain! Thanks

(Doug Dwyer) #2

Not quite sure what you mean by “…goes into its home.”


To slow down the speedo reading you add teeth…typically increasing the tooth count on the driven gear, not the drive gear. A rough rule of thumb is 4% per tooth.

Sometimes changing the tooth count on the driven gear mandates a change to the drive gear. This is due to the angle of the teeth.

Presumably you have an old BW transmission. I’m not sure what’s available out there in terms of speedo gears.

Another alternative is a ratio-adapter…a little gearbox that goes between the trans and the speedo cable that overdrives or underdrives the cable. These are fairly easy to find for the more popular and common transmissions but, again, I’m not sure about any BW particulars


(Frank Andersen) #3

I ‘think’ the mechanical speedo uses eddy-current, sort of ‘magnetic’ drive, Sawyer…

If so; there is a disc driven by the speedo drive, and a parallel , spring loaded disc, with the speedo needle attached. As the driven disc rotates, the needles disc moves against it spring - proportional to the driven disc’s speed…

If so; the speedo is calibrated by varying the distance between the two disks…

But Jaguar might have selected something more sophisticated…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(sawyer10702) #4

Doug… I meant home has where the drive gear which attatches the the Speedo cable goes into the transmission and a single bolt that holds it in.

(sawyer10702) #5

Frank i understand that part. But why the last trans which was from a xjs 85. Was fine no issues. I have a xj12 rear end. That is a dana unit. I think maybe that rear end with bw12 maybe the gear ratios are different! I’m not sure how this all works.

(Frank Andersen) #6

The pick-up point for mechanical speedos is usually the gearbox output axle, Sawyer…

Which then expresses how fast the wheels are turning, road speed - but that is of course is modified by the differential ratio. So principally a specific gearing of the speedo would be required to mate the speedo to the actual road wheel rotation. Which would likely be at the gearbox pick-up point - the gear driven speedo cable connection. Though it could be at the angle drive - or simply disregard effect of the different diff ratios…?

xj6 85 Sov Europe (UK/NZ)

(Doug Dwyer) #7

So, if I understand correctly, you changed transmissions?

When you change transmissions you get the speedometer gears that were appropriate for the original usage of that transmission…which might well result in an inaccurate speedometer reading when the transmission is installed in an entirely different car.


(sawyer10702) #8

Frank i used the original angle drive and the gear driven Speedo cable connection. So now you and Doug have me thinking??? So the angle drive is it possible i have the wrong angle drive? Are these drives differant on every early jags? I thought these were the same units.these units possibly headed also?

(sawyer10702) #9

Doug let me clarify i had a bw 65 that had been installed in my series 1 xj 6. But because it had a kick down cable that was not attached to anything it was not fun to drive. So I put the original trans back in bw12 which it drives and shifts normally now. So I did use angle drive and Speedo gear that came with trans. But has i said to Frank possibly I have something mixed up.

(Pete55Tbird) #10

Hi, your dealing with “old technology here” so ask an old guy. Your BW12 has a drive gear on the
output shaft . The end of the speedometer cable that goes into the angle drive has a driven gear.
The teeth (Number of splines) on the driven gear vs the teeth on the drive gear COMBINED with the rear
axle ratio/ tire diameter give you MPH on the speedometer. Try to find and change the driven gear to match your REAR AXLE RATIO. Pete

(Mike Badger) #11

When I bought my DDS years ago, (BW12 and 3.31 rear ratio I had a similar issue……. The right angle gearbox which exits the rear of the BW12 was a SMITHS part BG2402-08 whereas it should have been BG2402-07 ( which is a 1:1 ratio) when I replaced the -08 with the -07 it all came back correct. These part numbers are cast into the body of the gearbox but are very hard to read. There are other ratios in the range but I believe 4.2 and 5.3 BW12 needed the 1:1 ratio.

Hope that helps a bit!


(sawyer10702) #12

Ok pete. Will do thanks for the info.

(sawyer10702) #13

Mike i have another bw 12 and I took the drive angle drive from it and the number is a -07 and I checked out the bw65 that I took out and it also has a -07. So I will check the one out in my car has soon has the weather breaks it’s been coming down cats and dogs here in sunny northern California and more to come so I will let you chaps know what my findings are…cheers

(Rob Reilly) #14

Looking in my XJ12 S2 parts book, the part number of the speedo drive gear on the main shaft is C.29549 up to certain VINs, then after those VINs it is C.43913.
The driven gear is C.29550 up to those same certain VINs, then after those VINs it is C.43915 for 3.31:1 rear axle and C.43914 for 3.07:1 axle.
The rear differential identity can be confirmed by a letter S or X or no letter adjacent to the serial number on the nose.

(sawyer10702) #15

Rob thanks for getting the parts books out, good.to have some info! I need to get those xj12 books. Is there any download for this? I have the series.1 parts book. But no help in this case…cheers much appreciated.

(Doug Dwyer) #16

Here are older catalogs. Not sure if they’re downloadable or not



(sawyer10702) #17

Mr Dwyer.
You are a diamond! That’s is a British term for brilliance…thanks for that link. And yes I’m able to download and zoom in.
Cheers mate.