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MG MGB Technical - Overdrive conversion speedometer?

I found a nice late OD box for my 1970 B. I have the engine out for cam replacement, oil leaks, clutch, etc., and felt it was a good time to install an OD box.

Please advise if it is necessary to change the speedometer calibration. Mine was spot on with the original gear box. I have heard that the calibration is different for the OD box but it doesn't make sense to me. The speedometer is being driven at drive shaft speed, which will not change due to the conversion.

Let me know if I need to have the speedometer recalibrated and if so, if there is a gear type that I can cite to MoMa that will avoid counting cable revolutions of the cable in a 52 ft. run
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Glenn Mallory

If your O/D has a blue label then it needs a 1000tpm speedo.

Look at the face of your speedo towards the bottom. It will tell you its tpm.

There are plenty of 1000tpm speedometers out there and will be much less expensive to get one of those than having yours re-calibrated.
Steven Rechter

"Look at the face of your speedo towards the bottom. It will tell you its tpm."

As a 1970 it should say 1280, which will give significant under reading of both speed and mileage with a blue label OD. Assuming North American LHD spec you would need an 80mm speedo as used on rubber bumper cars before the 1977 model year. From 1977 on they were 4".

However if it is a black label that would originally have been 1280 and so directly compatible, excluding any changes that might have been made by POs to your speedo or the OD you have bought.
Paul Hunt

Paul and Steve,

Do you have any idea of the vintage of blue and black label Laycocks? The transmission is being rebuilt so I don't have immediate access to the unit to view the label. The OD transmission does have a dipstick (like the one that came in the car). My speedometer is a 1280.
Glenn Mallory

The black label was fitted to chrome bumper cars and the blue to rubber bumpered.

If it has a dipstick, it should be a black label unit, as rubber bumper models had a filler/level plug in the side of the gearbox.
Dave O'Neill 2

As Dave says if it has a dipstick, then it should be a black label, and hence compatible with a 1280 speedo.
Paul Hunt

This thread was discussed between 03/03/2016 and 04/03/2016

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