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MG MGB Technical - blue label o/d speedo gear in black label

I am trying to correct my speedometer reading.

I have a black label o/d attached to a 1040 tpm speedo that reads very high. (>10 miles)

I believe the transmission has a 1280 tpm gear installed.

Can I replace it with a blue label 1000tpm gear?
Steven Rechter

You can, but doing so will be a major undertaking. It would be much wiser to get your speedometer re-calibrated. After all these years, they tend to be off anyways. RAY
rjm RAY

Thanks Ray. Foolishly I was hoping for an easy fix.

The cost of changing the gears on the speed and odometer sections is $300.

However installing a speedometer ratio adapter in series with the speedo cable at the transmission costs <$100.

That leads me to my question in another thread: What is the size/threads of the speedo cable connection at the transmission? (I fear that it is Whitworth)
Steven Rechter

Stephen - The speedo cable threads is not Whitworth (Whitworth is a course thread). I would guess that the threads in question are going to be British Standard Brass or British Standard Cycle. I will see if I can determine what it is for you.
David DuBois

You are not finding Smith's cable ends anywhere common in the US. You can take the threaded bits off an old cable and the speedo shop should be able to reuse it with appropriate cable ferrules. I've done it.

A decent speedo shop can easily cut your cable at any point, and install the ratio box in the cable, with whatever standard ends are required to fit the box. Not in the tight quarters under the car either! I recently saw a picture of a ratio box fitted without cable ends - the cable was just cut and fitted with small hose clamps. These ratio boxes are common on big trucks, so look for a big truck parts supply place if you cannot find a speedo shop. Swaging the inner to make square drive ends is SOP at these places; I've done it with home made tooling.

Or get a correct used speedo and swap bits around to use those guts and your dial/case.

FR Millmore

FRM and others- I have the same issue, a blacklabel 1280 tpm OD feeding a 77 MGB dash speedo geared at 1000 tpm. Previously on a T-9 trans swap, I had the ratio reducer gearbox made, and it works great. I can do same here, but am wondering if the gut swapping on the speedos is viable. I mean, the 1280s I know are an 80 mm gauge with an odometer reset punched out below the housing. The 100s appear to sport odometer gears that are integrally attached to the odometer drive, rest through the face of the gauge, not the bottom, and don't seem to be at all to the guts or screw down needed by the 1280 speedo internals. If I can, I'd prefer to go this route. Any clarification for the internally murky? Cheers, VEM
vem myers

Have read a lot of articles, some with pics so not complete BS, about guys doing these frankiininstruments in assorted varieties. My working theory became ""look at what you got and what you need and see what you can do" - when you need it, because there are far too many variables to keep in your head.
Points of interest:
Instruments tend to come in time group families, and it may have little to do with size, case design or even car make.
Even totally different insides might be adapted to your case/dial, with an extra screw hole or such. Tachs and temp gauges are good for this.

There is at least one excellent internet article, and of course I can't find it or the link on my computer, but it is there.
Google "repairing Smith's instruments" or similar.
I think it is a Triumph guy.

But in your case I would think that finding the correct MGB speedo should be easy enough, given that it was the best selling sports car in the US. Not like you are looking for AC gauges, or Jowett.
If you get a not working one, the link you found in Google will probably clue you as to how to fix it!

And since you have revealed that you can get a ratio box fitted practically free and without hassle, why are you bothering me - or yourself?

FR Millmore

When I put a '72 Black Label OD transmission in my '67 B, I took the internals from a '72 speedometer and easily fitted them into the earlier housing. I had to drill one small mounting hole for the face to mount to the speedometer body. No big deal. I was even able to use the original smiths needle. It's extremely accurate and it looks bone stock. RAY
rjm RAY

Steven - The threads for the attachment of the speedo cable to the transmission measures 3/4 - 28 or 18.4 - 0.9mm (the metric thread gauge matches closer than the British or American thread gauges). Neither of these sizes show up in any of the thread charts that I have. Fletcher's suggestion of getting the parts off of a defunct cable is probably the best idea.

FYI - this thread fits the transmission of TDs through the MGBs (for whatever that is worth). Cheers - Dave
David DuBois

Thank you Dave. BTW I will be sending a fuel pump your way soon for an electronic conversion.
Steven Rechter

I would love to hire the green '66 MGB for a glorious drive down to the wreeiins for the the dates 19th-22nd may 2010.Is this available and what would the cost be?I wish I could own and choose to drive this sexy car everyday!Thanks,Madelaine
James Neel

This thread was discussed between 06/07/2012 and 22/07/2012

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