If your 122 is a wagon your rear end ratio is 4.56:1 or 4.10:1 for a sedan. The speedo drive in your M41 is probably for about 4.10:1 ratio, but it may be different, as I don't know what all was offered in 140's. That may account for the speedo error, or the speedometer itself may be the problem. The best way to check is if you can find a speedometer check section on a local highway and check how close the odometer is.

If the speedo seems to be ok then you could look for a speedometer drive for a lower gear ratio rear end.

Hope this helps.
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Lee