I suppose the way a cable would cause inaccurate readout would be is if it were broken and the driven end turned the other end slowly. That seems implausible to me though. Keeping the cable lubricated is important and often overlooked when bringing a car back to life after sitting for many years. I can’t remember what the appropriate lube is but I’d think it would be a mixture of gear oil and grease.
JB (and I) have electronic speedos. They are driven by “counters” that count revolutions. There are three types of counters that I know about. One (what I think JB has) that counts revolutions of the speedo gear in the tranny, another that counts revolutions of a spinning part of the car like a hub (IIRC, Clifton has this type), and a GPS antenna that translates distance into counts for the speedo. My speedo gear was eaten by my tranny so I chose the last type. Of course, it is useless in tunnels and dense tree cover. It also takes a couple of minutes to acquire satellites.
