Speedo reading half the correct speed

[Steve_S]

I have just changed the gearbox on my 948 cc Minor. The speedo now reads approximately half the correct value. It worked fine with the old box.

Any thoughts?

I have read something in the workshop manual about the speedo drive changing after a certain date. Could this be anything to do with it?

[win]

Hi
The early smooth case boxes had a speedo drive of 4.5 to 1, later ones 2.6 to 1.

Looks like your replacement box has the 4.5. You can check this by removing the cable, and counting the prop shaft and speedo drive turns.

Unfortunately you have to strip the box to change them, you can't just change the worm.

You could divide your speed and miles done by .57
Not sure, but, you may be able get a speedo with a TPM .57 % lower than your existing one, that should work.

Regards Win

[mike.perry]

The workshop manual gives a speedo ratio of 5/13 = 2.6:1. It does not give any other figures for the 948 box. I think that the 4.5:1 is for the Series 2 box. All the 948 speedos and the 1098 LCV which also has a 4.55 diff, have speedos with a TPM of around 1504. See list on Series MM website.
The Series 2/3 Parts List gives changes of speedos at chassis nos #290000 = Series 2 restyled inst panel, and 448000-579000 = Series 3.
Haynes lists the 5/13 ratio for all the boxes from 948 to 1275.
The only thing which would affect the speedo reading would be a change of diff ratio or tyre size, or possibly a change from an 803 box.

[bmcecosse]

You've hit the nail on the head there Mike!

[Steve_S]

Thanks very much for the advice so far.

So it seems that my 948 box somehow has a speedometer drive from a Series 2 (803 cc) box. Is this possible?

If so, presumably I can fit a speedometer from a late Series 2 if these are baically the same gold face speedos as fitted to 948 Minors.

Cheers,
Steve

[liammonty]

It's only possible that you've got a series 2 drive if someone has also changed the internal gear that the speedo drive meshes with as well- you can't mate the series 2 drive with the 948 'box, and vice versa- you have to replace the drive gear on the 3rd motion shaft (I think!) too. It's possible, but a bit of a nuisance unless you like stripping gearboxes. My series 2 car speedo, with a 948 engine and box fitted, read very high, which I think is the opposite problem to what you've got. I wonder if someone has replaced the tail piece of the gearbox of a series 2 with a 948 one in your case- it could be that you have a box with 803 internals that looks like a 948 box with the remote shift tailpiece.

[fussyoldfart]

Just to be certain of what is happening have you checked to see what the odometer records over a known distance? If the odometer is right the problem is in the speedo head, if the odometer records only a half mile or so per mile driven than the problem must be in the TPM of the cable.

[win]

Of course, Mike is right what should be there, but, if the diff and wheels have not been changed, It is quite possible someone has fitted a 4.5 to 1(2 start worm 9t gear) instead of 2.6 to 1(5 start worm 13t gear), the bore and width of the worm are the same, and it fits in the same place on the main shaft.


Regards Win

[mike.perry]

The tail section of an 803 box can be fitted to a 948/1098 box to maintain the the original lever when changing the 803 engine and box for a larger unit, so presumably speedo drives can be interchanged.
You could try removing the drive and counting the teeth which should be 13 on any box from 948 up.

[liammonty]

Mike- the speedo drives can be changed, but ONLY if the worm gear on the mainshaft (3rd motion shaft) is the compatible one, i.e. you can't change only the drive 'head' that screws into the box without also changing the worm gear- they won't mesh. This is what I was trying to explain, but Win put it better than I did. So, if the number of teeth on the drive gear is not what you've said it should be, then it can't be swapped for the correct one until the worm gear is changed too. I had this problem on my early SII car. As the OP suggested, I think an easy fix, if this is the problem, would be to just fit a late SII speedo, as that would give the correct speed.

[mike.perry]

I meant the drive and worm gear which I assume can be removed from the shaft?
On the Series MM box there were instances of the worm drive slipping on the shaft. Is this possible with an A Series box?
The late Series 2 speedo has the same 1000 tpm as the early Series 2 small speedo and would only be correct for a 5.375 diff.
Unfortunately the BMC workshop Manual does not give the speedo drive ratio for an 803 box. Is it listed in the Haynes Manual?

[liammonty]

The drive itself can be removed really easily, and the number of teeth could be compared to what they should be for a 948 / 1098 box. From that it can be determined whether the correct worm gear is in the box, as the later drive won't mesh with the earlier worm and vice versa (speaking from bitter experience having converted a 948 box to fit my early series II...). Not sure about the worm gear slipping- I think it could in theory, but that would probably give an inconsistent reading wouldn't it, unless it was slipping at a constant rate? Could be it though. Not sure if the drive ratio for the series II drive is listed in the Haynes, but I'm sure I found it somewhere when I was having problems with mine. And you're quite right- the drive, if working as it should, would be right for a 5.375 final drive, though in practice, I found mine over read so much when set up as standard (weak speedo springs, lower rolling radius radial tyres compared to crossplies etc) that it probably would be reasonably accurate, though the odometer might be a bit out. I had this trouble with an early series II, with the MM style speedo, though I believe all series II gearboxes and speedo drives are the same.

[Steve_S]

Looks like my best bet might be to get my original speedo recalibrated by a specialist firm.

Thanks for all the advice.
- Steve

[bmcecosse]

I doubt it - that will be very expensive.......

[win]

A crude way to determine what TPM you need, is as follows.
Put a mark on the rear tyre, in line with a mark on the floor.
Roll the car forward 1 rev, measure this distance in inches. Divide 63360 (inches in 1 mile) by the distance the wheel moved.
Multiply this figure by the diff ratio.
Divide this figure by 2.6, this gives you what your original TPM should have been,
or 4.5, this gives you what your TPM should be now.

(If you are not sure of the diff ratio, count the number of prop shaft turns, while the wheel made 1 rev. 5.375:1, 4.55:1 and 4.22:1 are standard ratios.)

Car moves 75" in 1 rev. (63360 divide by 75) = 844.8 multiply by 4.22 =3565.05 divide by 2.6 = 1371 TPM 1376 is a standard.
Car moves 75" in 1 rev. (63360 divide by 75) = 844.8 multiply by 4.22 =3565.05 divide by 4.5 = 792 TPM

Do the sums using your figures, you never know you might be lucky, failing that pull the box out, its not much of a job to change the speedo drive.
Regards Win

[mike.perry]

I had better put some new batteries in my calculator