Here is something that Margaret Lucas at Mo-Ma told me. Many of the
replacement speedometer cables have the instrument end of the cable cut too
long. The cable has a collar at the instrument end. The cable must not
extend more than 3/8" beyond this collar. She said that if the cable
extends more than that, it puts pressure on the part that spins. This is
more or less what is said below, but w/ a little more detail.
John Snyder
> If a speedo is reading high it usually means the guage itself is not
> working properly, was rebuilt incorrectly, or is out of adjustment.
>
> Another potential is for the speedo cable to be stretched/worn/too
> long, and thus it will put pressure on the back of the guage where it
> connects with the cable. If this happens, the cable can put some
> stress on the speedo instrument itself, causing the little magnetized
> wheel to bind with the needle magnet causing a high or jumpy reading.
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