MGB & GT Forum

1969 MGB GT, non overdrive car with black label overdrive gearbox from a 1973 UK MGB. 1280 TPM speedometer.

I installed the overdrive gearbox this winter. I had previously removed the angle drives at both ends after one broke and put in an overdrive speedometer cable instead. Following the gearbox swap, I had to reroute the cable through the firewall on the driver side instead of the passenger footwell where it was previously. Not paying attention, I put the speedometer back in the dash putting the cable at an extreme 90 degree turn near the speedometer end. Predictably, the cable broke after a few miles. Ordered a new cable, rerouted it with softer bends, and now my speedometer reads fast by about 10 mph, or reads roughly 60 mph when my GPS speedometer reads 48-49 mph.

My question is what could be causing the speedometer to read high. Could there be a mismatch in drive/driven gears in the gearbox? Did the cable break possibly cause damage to the speedometer? Something with the new cable? Was there a TPM difference between US and UK gearboxes? Any help is appreciated.

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1969 MGB GT

The overdrive transmission needs a different TPM speedometer than the standard 4 speed trans. I made the same change to an overdrive transmission on my '74 MGB GT. Others on here will probably chime in with the different speedometer part numbers and which TPM you need. I found overdrive speedometers hard to find. There are/have been shops that can modify your existing speedometer, but the cost is approaching or exceeding the cost of a new or reconditioned one. I was not looking for originality, as my car is a driver, not a show car. I solved the problem by installing a GPS speedometer from Speedhut. There was also a thread on here recently where someone 3-D printed a gear reduction drive for the speedometer to solve the problem.
Don

From the Library:

1968 through 1974 (chrome bumper)

Laycock Type LH OD
Rectangular shaped access cover
Oval clutch fork boot
Dipstick (for checking oil)
Black label on the OD solenoid cover stamped "22/61972"
1280 TPM for OD and non-OD
Speedometer drive gear (on the mainshaft) is blue
Speedometer driven gear (on removable drive housing) is white with 21 teeth
OD on 3rd and 4th gears

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I was addicted to the hokey pokey but I turned myself around.

prop-a-gan-da: When a British person takes a close look at something

The overdrive transmission needs a different TPM speedometer than the standard 4 speed trans.

Not necessarily. Black label/top fill gearboxes, overdrive or not, came from the factory with 1280 TPM speedo gears; blue label/side fill boxes came with 1000 TPM gears. These gears can be changed over the years, but must be changed in pairs; you can't just change the externally-accessible driven gear. You must also change the internal drive gear. Overdrives use different speedo gears from non-overdrives.
Usually the 1280 gears are white; the 1000 gears red. If your gearbox has the wrong gears you'll see a speedometer and odometer error of about 28 percent. You can confirm the colors by removing the driven gear and the steel "bearing" that retains it, but oil will run out. You can minimize oil loss if you raise the rear end of the car.

Changing the internal speedo drive gear requires engine out and overdrive apart...

Using 1000 gears with a 1280 speedo will result in a lower speedometer reading. Similarly, switching to larger diameter rear tires, including 15 inch tires, will result in a lower speedometer reading.The OP's issue is likely within the speedometer.


Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Thanks, Dick. You’ve confirmed my suspicions and provided some valuable information.

I did some speed comparisons at 25, 45 and 60 mph according to a GPS speedometer app. The car’s speedometer read 18% higher on average. Not close to the 28% difference you mentioned with the 1000 TPM blue label gears. That seems to eliminate that possibility.

It’s worth mentioning that I have been 185/70/14 tires on the car. I also have the SN-5227/06 speedometer.

Would I be correct in thinking that a 1280 TPM speedometer is the same in function as another, regardless of whether or not it was originally fitted to an overdrive car or not? Looks like I’m on the hunt for another speedometer or having mine repaired.

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1969 MGB GT

Chrome bumper cars from 1968 on all used a 1280 speedometer...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)