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The 1800 speedo is a British made Smiths, with a dial and a pointer. The 122 is a German or Swiss VDO with a revolving drum. While the basic principles of operation are the same, it is unlikely that the mechanisms inside are very similar.
There could be an internal problem but I suspect that one of the following parts has been swapped at some time in the past: Instrument cluster, drive gear, transmission (including drive gear) or rear axle. You didn't mention what body type your 122 is. If you have a wagon (with stock 4.56 axle ratio) and a sedan (4.10 axle) cluster has been swapped in, it would read about 10% fast. I'm not sure what the errors would be for other possible mismatches.
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