Rig up a small version, use short wires and have it down at the engine bay to check to see if the cable lenght is the problem?
Righty, i guess wire colour doesn't matter as they all connect to fittings on the back of the rev counter, connections are coil, ignition, lights and earth. the letters UCI are printed on the back. Looking into the length of wire issue, it seems that there are 2 schools of thought... people who say it does cause a problem and people who say it doesn't!
Erm - surely the primary question should be: Did VW ever fit Jaeger instrumentation? I thought VDO made them - MG's and such were usually Smiths - and Jaeger were Jags etc. Getting the idle in the ballpark figure correct does not mean that the rest of the refs are then correct as you climb the rev range - strikes me that an original owner has simply lobbed a tacho in that fitted the hole, so you are wasting your time somewhat.
I reckon you've got a current-sensing tach (has it got RVI written on it somewhere?), as fitted to a lot of old Brit cars. Have a look on some of the old MG forums for connections. Current sensors, as far as I'm aware, connect in series with the coil wire, rather than in parallel. It's unlikely to work properly as it is. Cable lengths I don't think are the problem. You could save yourself some hassle by buying a VDO or other voltage-sensing tach.
