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Mike, I'm not sure. But the coupling between the spinning cable is simply a magnetic coupling. I assume they magnetize the spinning "rotor" to a specific strength. That spinning rotor sits inside a shallow aluminum cup with a pointer on the end of its shaft. And there are springs pulling the cup and needle back toward zero. I think the springs position is adjusted to achieve calibration, but it seems to me that the springs lose some strength over time, which can cause the needle to read a bit high. I assume the speedo can be calibrated by moving the spring position, but I have no idea how much. On my own speedo, after I installed an OD and changed rear axle ratio, my speedo was reading low by about 30% . I swapped some gears on an extra speedo I got from Jeff Howarth to get the odometer correct, and then I glued one or two tiny rare-earth magnets to the "rotor" to get the coupling increased and the speedo correct. First I did some road tests to establish how far off the speed was, say 27%, then I used an electric drill to spin the rotor. I experimented with different locations for the magnet, using just the magnet's holding power. Once I found the magnet position that came pretty close to increasing the reading, using the drill, by about 27%, I used super glue to permanently anchor the magnet in place. Amazingly, the speedo reads about right on the road. The odometer reads low by 2.5 % and I use that factor to correct my trip mileage and MPG calculations.Good to know. With speedos varying on the revs per mile, e.g. from 980 to 1504, do they just use different magnets and springs to deal with the different cable spin rate?
Mike, beamdream, Just FYI, changing the gearing inside the speedo will only change the Odometer readings. There is no gearing involved in the actual speedo MPH indicator, just magnets and springs acting on the the needle movement.
I realize this doesn't solve your problem, but given the scarcity and cost of the proper OD transmissions, it would be interesting to learn what your car has, assuming it isn't stock. Once you figure out how to get the speedo to read correctly, your car could serve as a model for others. Of course, there is no guarantee that whatever transmission you have is available in other countries, but it would still be worth knowing the details.
I realize this doesn't solve your problem, but given the scarcity and cost of the proper OD transmissions, it would be interesting to learn what your car has, assuming it isn't stock. Once you figure out how to get the speedo to read correctly, your car could serve as a model for others. Of course, there is no guarantee that whatever transmission you have is available in other countries, but it would still be worth knowing the details.
So an update on my quest to get the Speedo reading right, took the speedo apart and guess what it's been modified, see the first photo with details scribed on the rear of the dial face, the housing also has 1683 scribed on the back so assuming that's the actual tpm rather than the original 980 tpm as stated on the dial face, which would be correct for S1 with od.
Now here's where it gets messy, having done the measured 100ft exercise to get the actual cable tpm I find that it's actually 2138 tpm so goodness only knows what the od output pinion ratio is. In researching most of the Rootes products and the various speedo application's, the highest tpm number I've come across is 1580.
I might cast my search for this type of speedo a little wider and look at MG,s, Triumphs and the like.
Beamdream,
Please recheck your TPM number. If your speedo needs 980, or even 1683, TPM's and the OD is supplying 2138 TPM's, the speedo should read HIGHER than actual MPH. 2138/1683 = 1.27 or 27% too high.
I'm guessing you have a tire diameter near 24 inches (sorry, I'm not metric). That would give about 840 rotations per mile.
840 * 4.22 = 3545 driveshaft rotations per mile
100 / 5280 = 0.0189
3545 * 0.0189 = 67 driveshaft rotations in 100 ft.
To give 2138 TPM's, your cable needs to rotate 40.41 times in 100 feet of travel.
For 1638 TPM's, your cable should rotate 30.96 times in 100 feet of travel.
For 980 TPM's, your cable should rotate 18.52 times in 100 feet of travel.
All numbers subject to change depending on real world conditions.