Talleyman

http://www.gaugeguys.com/ratio-adapter.html
These guys carry the metric adapter to install inline to your speedometer to adjust for % change due to tire size change, et al. This is what we rally guys used (yet not as indestructible) for gear, wheel & tire change prior to 1976 or the pre-pc (personal computer) era, when people knew how to power shift and slide, oh I reminisce (LOL!!!)

Tiger03447

In some speedos that I've had in the past, the needle is on a spring and is not hooked to the cable at all. The cable carries a disc with a magnet that whirls around behind the needle. The faster the disc whirls, the more the needle is advanced across the face of the gauge, indicating a higher speed. This may not be true for Porsche speedos, but that's the way they worked a while back in the dinosaur days. Just my .02 cents..perhaps some insight?

Talleyman

In my experience the cable dictates the rotational velocity of the magnetic disc, ergo, change in the ratio of cable causes the magnet force relative to the spring to be altered. However, the best, albeit, permanent solution is to gradually change the magnitude of the disc. It is a trial and error approach. On mechanical speedometers in rally cars the ratio adapters were and are a quick, inexpensive fix. In fact, on my '66 912 changed to 15X5.5 wheels with -28% difference in speed indicated I was able to acquire a reading of negative 1-2% which imho is livable. We did use GPS to establish differentials.
Hope this helps...also there are other manufacturers of speedometer ratio adapters which might be more applicable. These guys have great customer service and are quite knowledgeable.