TwinSpark speedo problems

[Grange99]

My 1989 Twinspark 75 appears to have had a 3.54 diff installed at some stage in its life instead of the standard 4.1 diff, as my speedo readings are well under what you would expect - like 16% less. I proved this with GPS after getting a speeding ticket, and earlier sometimes having wondered why other traffic on the Expressway was always so slow.
My speedo had been fiddled with as well, to confuse things, and read high at lower speeds, about right at 70 to 80 km/hr, and slow at 100 km/hr plus. I have now reset the speedo needle so it reads correctly for a 4.1 diff, but would really like to have the correct speedo readings.
After discussing the problem with a few people I bought a Jaycar speedo conversion kit, got a mate to assemble it, and connected it up, but with no results at all. I have now been advised that the Jaycar kits do not work on Alfa 75 speedos, and I need a 3 litre instrument cluster to resolve my problem.
Any comments appreciated.